Walter,
Looks-wise, well, that's very subjective. On being a real sports car though, as an owner of C3, C4 and C5 Corvettes, there's really no comparison. The C5 is really a world-class car, whereas the C3 and C4 are just the best of the contemporary American cars.
C3s are almost unusable as daily drivers by modern standards. They are brutes (especially the early ones.) No sophistication whatsoever. Steering and brakes are numb with power assist, or unusable in traffic without power assist! (I've had 'em all ways - with and without.)
It's very easy to dismiss a C5 until you actually drive one. Even the most anti-Corvette people I know have done an about-face after driving my C5. It's the detail things that American cars never got -- like easy to read gauges, and a shifter that is so close in promimity to the steering wheel you can actually shift with a thumb still on the steering wheel. Also, the heads-up display is a plus.
When my '99 drowned in Houston, I actually went through driving all convertibles under $100K, and there were four sports cars that were truly great: C5, Porsche 911, Porsche Boxster S and Mazda Miata. (The BMW M3 Convertible had just come out, but were unobtainable -- going for $75K USED!) I didn't fit in the Miata or Boxster S, and the 911 was twice the price of a C5 convertible ($50K vs $100K) when realistically equipped. I also looked at a Honda S2000, which was fun but it wasn't really a usable daily driver.
The 911 and C5 were really equals in every way shape and form. In convertible form, the C5 is a better performer than the 911 ragtop (which is a bit heavier.) While everyone knocked the C5 interior, it was absolutely on par with the 996 911. It came down to twice the price for a marque that's nice...So I bought another C5.
Now on the Mustang...
I don't want to offend MGA Steve, and he already knows I love the early Mustangs, but the new Mustang Cobra won't do anything for me. It's not that I'm a snob, but it simply is a little too brutish, and nowhere near sophisticated enough for me. Something about a car that is fundamentally shared with a V6 secretary's car. I guess that's because I like sports cars the most -- and sports cars, by nature, aren't sedan-based, which is what the Mustang is. Even suping it up to the max, like in the SVT Cobra still doesn't change it's design for a small engine and commuting comfort.
I want performance in a sports car, and sophistication in a sedan. I don't think you ever get a world-class result by trying to eek world-class performance out of a sedan...mostly because you have to reduce sophistication to get it.
Which, I suppose which is why I'd choose a AC/Shelby Cobra over a Shelby Mustang, or a '69 Corvette over a '69 Z28 Camaro.
Yes -- I'm going on and on again.