I think the 2K1 Celica will respond better to mods. You also have the benefit of having a new platform that aftermarket companies are still developing for. Contrast that to the Prelude - hell, Honda doesn't even make our OEM kits anymore. At least let the body get cold first
As for the 2K2 Celica, I have also heard stories about the decreased redline and performance.
But if you get a Celica, you better get the GTS and you better get it in stick - the autos are pretty damn slow from everyone I've talked to. The styling is pretty questionable to me, I think the back end looks pretty fugly. It's a very nice car in blue or black though.
I've heard it's hard to keep the engine in VVTL-i or whatever their VTEC equivalent is, but that depends on your driving skillz...why don't you take it out for a spin and see how it feels?