I'm not going to tell you whats right or wrong on brand choices, but I will tell you, if you're seriously considering a used C5, go with at least an 01 model. They made several running changes in 01, especially with the block and motor. All 01 and later C5s, thanks to the release of the Z06 in 2001, got the LS6 block and LS6 intake. They definitely run a little stronger.
If you still want a pre-01 C5, I'd probably recommend a 99 or 00 FRC model (thats the same body as the Z06 became in 01). That car is considerably lighter than the hatchback coupe, and runs stronger. The chassis is also stronger.
I think performance wise you will be happy, but there are things I really hated about my Vette. Visibility is really poor trying to drive them, only 2 seats so you cant take the kids with you, and I always felt nervous taking it anywhere - so it became more of a chore. The cobra is a little less flashy (people think its just another mustang), has a back seat, and they got the perfomance. My .02
You can get a nice early ZR1 for $20k if you are patient. I like the 5.7L, 32v, DOHC motor. Those suckers sing! But the ride is rough, the drive train is noisy, the cockpit is cramped, the factory stereos don't work, the headlights are destined for failure, etc.....it's a GM in other words
Im one of those that think that a Mustang cant corner, brake or handle high speeds as good as a Corvette. So you can soup up a Mustang as good as a Vette but its hard to make it as good as a sports car. So yea, go with the Vette if your not brand loyal.
I like the vette but I am a brand loyal person and would never drop the money on a vette even if I had it. But everyone is right its a great SPORTS car, but its not a muscle car.