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.