Can't really go wrong with either a GS or Z06, as long as your first mod is a tire swap. Seriously, the Goodyears on the C6Z and GS are crap for the track. Frankly, I'm amazed the Z06 did so well in skidpad tests and such in magazines.
I'd say go with the Grand Sport. It's easier, less expensive and less risky to mod, and you can get a better deal on a low-mileage example. The only reason I bought a Z06 was because I knew the guy who was selling it and I got the "friend rate". But stock for stock, it's almost impossible to beat a clean used C6Z.
But whatever you do, don't pull the trigger until after the C7 is officially on the market. When that happens, it's pretty much guaranteed that C6 values are going to drop like rocks and dealers will practically be giving them away, though the 427 Vert will likely retain its value better than anything else. I mean, you can already get relatively low-mile(under 10k)ZR1's for less than $80k, so just imagine the deals you could get on a Grand Sport once Stingrays are on the street.