Losing: cargo space, mileage, people space, 2 doors, adjustable steering column, helluvalot of money, options for leather and such, probably going from no-service timing chain, build quality?
Gaining: bigass sunroof, 10-50HP (on a camry engine), newer car
Looks: The tC looks sorta silly in my opinion. just kind of a very plain "hi, I have 4 wheels, two doors, and a glass roof"... they also don't carry the rear side windows back to a point and thus have way too much sheet metal going from the trunk to the roof.
Feel: Mazda put a lot of thought into not making the 3 feel cheap... and the scion seems like it might suffer the same "you can so tell this was a $12,000 car with $5-18K worth of performance added" effect that befalls the lancer evo, impreza sti, and neon srt-4. It also has no track record yet for handling... My friend who deals in toyotas and scions is actually drooling over the mazda3 for its looks and handling.
Warnings: Chevrolet (cobalt SS) and saturn (ion redline) are getting a super duper overpower compact cars... what's to prevent you from wanting one of them when they show up... or a Mk V GTI... grass is always greener... (blah)
But ultimately: wait until the things get here so that you can test drive it and see if you actually feel that the extra HP is worth the losses... not to mention better quantify the gains and losses. Waiting will also give you a chance to let scion work out all the first model year kinks if you do decide you like it.