Here's something to consider:
The '03 Cobras tip the scales at almost 3800lbs.
While a C5 Coupe comes in at 3200 lbs.
If you were remotely interested in a turning contest, the C5 w/Z51 suspension has the Cobra beat easy. But since you're not...
Pulley'd, intake/headers/exhaust '03 Cobra will make ~450 rwhp. In order to equal that kind of rwhp/weight ratio in the Vette, you'd have to coax about 380 out of the LS1. Its not hard to do, but its more expensive. Ultimately, the C5 will have more go fast potential based purely on weight.
But I'd say go with the Cobra, even though I consider the C5 a superior car. You really can't argue with 450+ rwhp for a few grand worth of mods.
///M-Spec