Spacecowboy:
FWD A4's only have Front antiroll bars. Yours will probably have the thin 25mm one, where as sport models (irrespective of the engine) will have 29mm ones.
The only downside is that while the larger front bar will dial out some roll, it will increase understeer, as you cant up the rear to compensate. It will most likely make the car feel better near the limit, but it will also most likely reduce total grip.
I'd be doing dampers and springs first, and going from there
Jonion: Neuspeed do a 19mm rear bar, and they're the only company that does one any more. Whiteline used to do one (also 19mm and also rear only) but they've discontinued it. Most other suppliers seem to provide a kit with both front and rear bars, usually upgrading the front by 50-60% and the rear by a HUGE amount to compensate (like 150%) but i cant help feeling that this will only result in a very unsettled car over anything other than glass smooth roads.
I personally suspect that the difference is simply that whiteline (and neuspeed) produced a product that was actually designed to maximise the cars handling, and they've correctly realised the car really doesnt need any more front ARB, something that audi also realised, given the S4 and RS4 also kept the same 29mm bar, and the quattro gmbh upgrade for the RS4 was also rear only.
The other firms are pandering to people who dont really have a clue about suspension design, but who think more is better, and are therefore providing two bars must be better than one. They are still addressing the stock cars balance issue, by upgrading the rear bar by a lot more than the front, but that doesnt get round the fact that the unneccesary front bar upgrade will have negative effects on the car.