Personally I would rather have a 2005-2007 Corvette for that little bit more money. To me the C5, while having great exterior lines, had one of the worst interiors I have ever seen to the point where I would consider even a cloth Camry an upgrade! While the interior of the C6 is no award-winner, it is at least not so cheesy that it is distracting with rattles, excessive wind/road noise, and feel. A base model C6 will handle pretty much just as good, has just about he same power that can easily be bumped up well beyond the C5 Z06, and throw in a targa-top, and you got yourself a hell of a weekend car that you will also want to take out every weeknight you get the chance to. The C5 Z06 is a thrill (though not as much so over the C6 as you would think), but can quickly get your nerves with its shortcomings.
As for the other two, Mustang's have a clutch that is so heavy and a chassis that handles so horribly, it boggles me why in the world you would ever consider one. The GTO is a great car with little short comings, but given a choice between it and the Vette, my choice is an easy one. Just think to yourself, you are at the bar, and someone asks. Which would sound and look better, 'I have a GTO', 'I have a Mustang SVT' or 'I have a Corvette'.
Me? I am a BMW guy and given that money, would without a doubt get a BMW, but I know how to work on them myself and don't mind the extra maintenance; don't let people fool you either, they are a little more expensive to maintain and it can be hurtful if you don't know what to expect. Even being a diehard BMW fanatic, I have always had a soft spot for Corvettes.