I have tried both a front bar and a rear bar. I ran the rear bar when I was in ST. When I converted back to stock class, I switched to a front bar, b/c a rear bar wasn't allowed at the time. I ran a progress 22mm rear sway bar. I now run a 26mm front sway bar (stock mazdaspeed3 bar with poly swaybar bushings).
I'm one who has never really had a lot of complaints about getting my FWD car to rotate, so take this advice from that perspective. I prefer the improvement in tire wear from the front bar, and feel that b/c its such a nose heavy camber challenged FWD car it improves front end grip considerably, as opposed to making the car pushier like a larger front bar can sometimes. The conventional wisdom is big rear bar, but for a camber challenged, nose heavy strut car, I think it might be different.
That said, the fast people tend to run a rear bar. The guy I bought my wheels from, Clay McCool, said he couldn't stand running with a big front bar though, and he was very competitive (trophied in a 3 in H-Stock at a tour event). Feinberg runs a gigantic rear bar on his integra and picks his inside rear tire a foot off the ground (although on a double wishbone front suspension, going to positive camber isn't as big of a concern for him). My recomendation, drive my car, and see if you think its too pushy. If so, get a rear bar, if not, get a front bar. My car isn't stock legal, but the only things that make it illegal are the intake and the wheels being too wide.
The current running debate on stock class swaybars for mazda3's. I will warn, a lot of the opinions in this thread are flat out wrong, so read at your own risk. . .
http://www.mazda3forums.com/showthread.php?t=396129