I test-drove a Scion tC with my bro (he's looking to get himself a car) and I own a Mazda3s.
The Scion tC is a little more preformance-oriented, has a (much) wider range of options which can be added any time at the dealer, has the nice double-sunroofs, and is more spacious than any other 2-door couple I've sat in (front seats and back). However, Scion "pure-pricing" means you have to pay whatever is on the sticker (no negotiating), and I had some minor complaints about the ergonomics: the glove box and cup holders are a little hard to deal with if you've got someone riding shotgun, and its damn near impossible to check your blind spots through the postage-stamp sized rear windows. Not to mention, a large number of Scion options are pure rice (factory ground effects, multiple available rear spoilers, glowing blue lights everywhere) and some are overpriced and over-the-top ($1500 for 18" rims and high-preformance tires... on a FWD car). I have no idea what the supercharger option will run or what kind of performance-boost it will give, but that remains to be seen.
The Mazda3s sedan has a slightly nicer interior IMHO (more elegant, not so glitzy-looking), has a layout as good as any other small sedan I've ever sat in, and an excellent safety rating for a smaller car. Its also slightly more fuel-efficient, you get 4 doors (and decent sized rear windows), the engine has as much HP as the Scion, and there's a decent selection of options that are reasonably priced and don't go over the top (for example, the 3s wheels can be upgraded to 17" alloys with V-rated tires, which is probably the outer edge of what's comfortable to drive on a daily basis). You can also actually negotiate the price a little, so while the Mazda is more expensive when identically equipped, you can still get a dealer discount. No factory supercharger though.
Given the choice all over again, I'd still pick the Mazda. The Scion still feels like a purely marketed car... something Toyota pitched to compete with the Civic Si and Cobalt for the "youthful" market. The Mazda3s is just a car that you can love. Not to mention, it looks absolutely great on the outside: people stop me in parking lots to ask about it, and I still catch people walking around it later to get a good look. Not bad for a sub-$20k car.