In the realm of international relations, arms races are a recurring and potentially devastating problem. But the automotive equivalent -- the horsepower war -- just makes life more fun for enthusiasts everywhere. German automakers in particular have been trying to outgun each other at the dyno since the 21st century got underway, and the battle has really heated up over the past few years. The 2010 Audi S5 is a calculated addition to Audi's arsenal, a high-performance luxury coupe/convertible intended to counter BMW's growing stockpile of turbocharged sporting machines.

When the strikingly styled S5 debuted two years ago, it was available solely as a V8-powered coupe with either a six-speed manual transmission or a six-speed automatic. The still showstopping coupe soldiers on for 2010, but it's joined by a soft-top cabriolet that comes with a different powertrain: a supercharged 3.0-liter V6 paired with a seven-speed dual-clutch automated manual. Shared with other performance cars in Audi's lineup, the cabriolet's force-fed V6 makes 333 horsepower and as much torque -- 325 pound-feet -- as the coupe's 4.2-liter V8.

If you're in the market for a performance coupe or convertible, you'll also likely be checking out the S5's natural rival, the BMW 335i coupe and convertible. In comparison, the S5 coupe's V8 has a far more engaging character than the 335i's businesslike, turbocharged inline-6, enabling the Audi to serve as a budget-priced alternative to top-dog luxury coupes like the BMW 6 Series -- and justifying its considerably higher price. For the cabriolet, though, the BMW's comparable engine and retractable hardtop make the Audi's price premium much harder to swallow.

Still, the 2010 Audi S5 has a trump card that the Bimmer lacks, and that's its drop-dead gorgeous styling. For coupe buyers, it's an additional reason to feel good about spending a little more; for Cabriolet buyers, it might end up being the deciding factor. And let's face it: When you're dropping $50,000-$60,000 on a performance car, looks are important, and by this measure the S5 stands alone. We'd strongly advise that you check out the 335i (or the coupe-only 335xi if you're high on the Audi's standard Quattro AWD) as well as Mercedes-Benz's new E550 coupe, but we wouldn't blame anyone for being seduced by Audi's stunning intercontinental two-door missile.

The high-performance 2010 Audi S5 is available as a coupe or soft-top convertible (the cabriolet) in two trim levels: Premium Plus and Prestige.

The base S5 Premium Plus comes standard with 19-inch alloy wheels, a sport-tuned suspension, xenon headlamps, foglamps, power heated exterior mirrors, rain-sensing wipers, an electronic parking brake, a multifunction leather-wrapped steering wheel, a panoramic tilt-only sunroof, tri-zone automatic climate control, power and heated front sport seats, napa leather upholstery, a 50/50-split-folding rear seatback, Bluetooth, Audi's Multi Media Interface (MMI) with dash-mounted controls and a CD audio system with satellite radio and an auxiliary input jack. The S5 Cabriolet adds a power-retractable soft top and a wind blocker.

The Prestige upgrades to keyless entry/ignition, auto-dimming mirrors, a color information display, a Bang & Olufsen sound system with a DVD player, driver memory functions and a hard-drive-based navigation system with real-time traffic updates and the third-generation MMI system (including revised menus and a joystick-like button atop the control knob).

Optional on the Premium Plus trim are the navigation system and Bang & Olufsen sound system. The S5 Prestige is eligible for adaptive cruise control, a back-up camera with rear parking sensors (included with the navigation bundle on Premium Plus), a blind-spot warning system and Audi Drive Select (includes adaptive suspension dampers, variable-ratio steering, a sport rear differential that varies torque between the rear wheels, and four selectable driving modes for transmission and steering response and ride tuning).

All S5s can be outfitted with a Sports Rear Differential package (essentially Drive Select minus the adaptive dampers and the variable-ratio steering), various decorative inlays and Alcantara seat inserts. The cabriolet-only Comfort package adds a neck-level heating system and ventilated seats with special Milano leather upholstery.

The S5 features two engines: the coupe's 4.2-liter V8, which churns out 354 hp and 325 lb-ft of torque, and the cabriolet's 3.0-liter supercharged V6, which is good for 333 hp and 325 lb-ft of torque. A six-speed manual transmission is standard on the coupe, and a six-speed automatic with manual shift control is optional. On the cabriolet, a seven-speed dual-clutch automated manual is the only available transmission.

In performance testing, we spurred a manual-transmission 2010 Audi S5 coupe from zero to 60 mph in a fleet 4.9 seconds.

EPA fuel economy estimates for the V8-powered coupe stand at 14 mpg city/22 mpg highway and 17 mpg combined with the manual transmission, while the automatic is rated at 16/24/19 mpg. The V6-powered cabriolet comes in at 17/26/20 mpg.

Antilock disc brakes (with brake assist), stability and traction control and front-seat side airbags come standard on all S5 models. Coupes have side curtain airbags as well, while cabriolets have active roll bars behind the seats.

The 2010 Audi S5 is a sharp-handling car, with tenacious AWD traction and little body roll to speak of. It's also rather heavy, however, and feels the part. In other words, it's a classic grand touring car that happens to handle extraordinarily well. The standard 19-inch tires give the S5 a firm ride and notable road noise over some surfaces, but neither trait is objectionable by the sporty standards of this segment. The S5's standard speed-sensitive power steering system, on the other hand, feels artificially light at parking-lot speeds and too heavy on the highway, though it's precise and responsive in quick transitions. The optional variable-ratio steering system that comes with the Audi Drive Select package provides a similarly contrived feel.

The coupe's 4.2-liter V8 is simply one of our favorite engines. Acceleration is authoritative, yet it never seems to be working hard, emitting an intoxicatingly mellow burble from idle to redline. The manual transmission's clutch is sometimes tricky to engage smoothly, but the shifter is precise; the automatic is less involving but a perfectly adequate alternative. The cabriolet's supercharged 3.0-liter V6 is smooth and strong, and its automated dual-clutch manual is quicker than you'll ever be in a conventional three-pedal car. However, the V6 lacks character compared with the V8 -- it's highly capable but hardly inspiring.

The S5's attractive and high-quality cabin is one of its strong points, though the competition has largely caught up to Audi's formerly segment-leading interiors. Audi's proprietary MMI routes many functions through a control knob mounted on either the dashboard or the center console, depending on whether the optional navigation system is present. The dash-mounted version can be somewhat frustrating, but the console-mounted one is a different story -- thanks to Audi's latest third-generation menu structure and a special joystick-like button atop the control knob, this is the most user-friendly MMI yet.

The front sport seats are excellent for both enthusiastic driving and long-distance cruising. Unlike the A4 sedan on which it's based, though, the S5 features a low seating position and a high cowl and beltline, which is sportier but may make shorter drivers feel submerged. The rear seat's limited head- and legroom make it suitable for small passengers only, though people stuck back there will be treated to their own set of climate controls. The rear seatback folds down in both the coupe and convertible to accommodate larger items, a welcome convenience in a high-buck performance car.