I would say go with the Si. Like it has been said before the car has more aftermarket support then the Tc does. Also every test that i have read about the TC they say that you can really feel the weight of the car.The honda also has a bunch of engines that you can swap in to it.
Now to play devils advocate what does the Tc have in its favor? Well for one thing it has a factory forced induction upgrade(Oem reliablity and no dealer hassles). Its heftyness can be counteracted by a Carbon Fiber hood, Lighter wheels/Tires etc. Also it seems with the Scion line up Toyota has been very open about aftermarket support. They gave preproduction Xa and XB's to companies like AEM so that Cold air intakes and such could be available when the cars were released.
So basically they both have good things going for them but i would go with the SI