If you just upgrade rear sways, there will be more oversteer.
If you just upgrade front sways, there will be more understeer.
You might want to look at upgrading with factory goodies. For your car, they have the front E30 Cabrio bar (22mm) and the E30 M3 rear bar (14.5mm). However, I'd just upgrade the rear bar and see how you like that, because installing both might offset your handling dynamics and give you more understeer.
they are 25mm in the front, and 22mm in the rear, and they are freaking awesome, and you really really feel the difference. Ive heard only good things about them, and they are like 425$ for the set i believe, and it comes with all the parts(ofcourse)
"You've got to fix the nut behind the wheel before you start fixing the bolts on the car"
Proud member of The "First 10 Members Playaz" Club
Founder and Proud member of The "Ran from a Cop and Got away" Club (x6, and counting)
Your car already has sway bars. They really aren't that thin. No, they are not worth it as your first suspension modification. You should save up for springs/shocks or coilovers if you really want to feel a difference. I added aftermarket sway bars after my shocks and springs and barely noticed a difference, but rather lowering the car with stiffer shocks helped far more than anything i've ever added. A strut tower brace is not worth it as a first modification aswell. Beware that stifferening up the body too much can actually hurt your performance. Also, the only truly effective strut tower brace is one that is welded to the strut towers.