Well, a better question is street car vs non street car. Street cars doesnt have to be street legal though.
A street car is a semi sport car so to say, even a high hp semi sportcar can be a streetcar. For example impreza, evo, rx7, skyline, supra, s2000, camaro, etc.
Supercar on the other hand, is cars that was build from the manufacturer to be unique and exclusive. For example ferrari enzo, gallardo, zonda, mclaren etc.
Streetcars can perform better than supercars, but they were build for different customers, intensions etc.
Depending on were you live, street legal cars is hard to define. Most of the modified cars arent 100% street legal according to the law, but it depends on how picky you are. Personly i dont care about the law or society, or human right (just kidding)
A street legal car in some states in US might aswell be illegal to drive on public roads in sweden, and vice versa.
But if some one wants to race me in his 900 hp enzo, and i have like a camaro with 660hp? Is that equal according to you? And that is a much better question. People in gt5 dont understand that weight, hp/tq curve, aerodynamics etc, must be some how equal if there are going to ba a fair race (drag racing).
"ohhh i smokes his supra, camaro" - I hear that often, but they have cars that has more hp, less weight, and better aerodynamics.