Maybe this isn't a fair comparison since the A3 is $5,000 to up to $10,000 more, but what the heck. I haven't seen the A3 0-60 numbers but with about 50 extra hp it must be quite a bit faster. The Mazda3 is a sub 8 seconds. For me, I'd take the Mazda3 for the money and beef it up a bit. Put in a cold air intake and maybe a performance exhaust and you've got a real rocket for a great price.

Here is some Audi and Mazda info

Audi A3 Review
http://www.automedia.com/rts20050701a3/a/rts20050701a3/1

Audi A3 Specs
http://www.automedia.com/nvg/test-b2006-Audi-A3-M.asp

Mazda3 Review
http://www.automedia.com/rts20050401m3/a/rts20050401m3/1

Mazda3 Specs
http://www.automedia.com/nvg/test-b2005-Mazda-MAZDA3-SH.asp

Mazda3 Video
http://www.automedia.com/featured_vehicle_main.asp

i say go with the mazda 3. it has alot of features for the money.
and if you want something with more power i wouild wait for the mazdaspeed 3

i go with the 3 because it will cause less to maintain. but becareful of the aftermarket mods it will set your MIL light if inproperly done. just find a ford focus with mods it the same car, the 3 might be heavier. moded svt focus ran 10.3 best in the 1/8. lol fast my ass.