RSX Type-S is 23,500 including leather, moonroof, 6 CD changer, and pretty much every option standard. The only options are cosmetic body peices which don't affect performace at all. The RSX is hardly larger than the old integras- still a compact.
The Celica definitely has a more aggressive appearance though it does border on boy racerish vs the understated RSX exterior- its up to personal preference though I do prefer the RSX exterior.
Interior no doubt goes to the RSX- more roomy, more features standard, better ergonomics, far more luxurious feel. However the Celica still has a top notch interior with the excellent Toyota materials you expect.
Power to weight ratios are pretty much equal so full throttle starts are in essence equal- it'll come down to the driver if both cars are driven equally.
However due to the its larger displacement and i-VTEC, the RSX has more usable torque lower in the rev range, making it easier to drive 6/10s in, much easier around the city. Also the 6-speed in the RSX is easily superior to the notchy GTS 6 speed gearbox.
Where the celica shines is in handling- with more aggressive tires, lower COG, superb suspension setup, the Celica has superior handling and response. Though I must throw in- the RSX is no slouch and a $200 spring kit(Comptech baby) does do wonders to the Type-S.
Overall, if you get the options in the GTS to bring it up to the standard Type-S level, the prices are pretty much equivilant. Both are excellent cars, its really a matter of personal preference.
I bought the Type-S b/c I felt it was a better combination of sport and luxury than the GTS and at my age, I want both.
Review including GTS, RSX, Eclipse, GTI