FWIW C5s are drive-by-wire too. But there’s ZERO lag. When ppl drive my Z they often comment on how good the throttle response is. It’s also easier for tuners to deal with big cams since they can control the closing rate of the throttle body.
To the OP, if the car is just going to be a “toy” not a DD there is absolutely NO question: C5. Get a FRC if you can find one in your price range, I just like the FRC/Z06 roof-line a lot better. For the same HP a C5 is always going to be faster than a GTO being 600-800lbs ligther. They handle a ton better. They fit way more tire. AutoX or Road Course worthy in nearly stock form (just good brake pads and fluid, if you go to R-compound tires toss in some ducts). Overall a used C5 is simply the best deal going in used sports cars these days. If you want to AutoX competitively spring for a Z06. With a C5Z and a few relatively cheap mods (front sway bar, shocks, tires) you have “the car” for SS that would be nationally competitive if you’re up to it.
-TJ