From my experience i believe that a Revo chip works realy well. The thing is that all this depends on how much money are you willing to spend for a single chip on your car. It would always be safer to go for an MTM one which always comes with Gurantee. ATB is good as well and they are know to give out some very good nubmers in hp. As for mentioning Dahlback before i would not recommend them as a guy i used to know had his ECU send over for a remap session and although they send the ECU back to him it wasn't working. And if that wasn't enough he had to send it another 1 more time to get it wrong again and then decide to take it to a local proffesional in his country....
Another very good brand for chiping the famous 1.8l t engine is Oettinger. All these companies have really small differenses between them chips. One of my friends ownes two Audi S3s and one has an MTM and the other has an Oettinger one. The mtm is much faster at low rpms although the Oettinger caches up after 200kmh and keeps on pulling hard until 280kmh(at least on the dash)What i am trying to say is that it is really subjective and it is up to you to decide the branding. It is all pretty much the same thing with similar pricing. Just make sure to choose from well know companies inside the German Tuning scene. One other upgrade worthing its money would be a swap for an S3 Turbo. It still quite small but that keeps the price down and it is capable of producing a well 320 hp.
Last thing: if you decide to go for the chip it would be nice to put some silicone hoses as well (Samco)because of the high boost the turbo is going to produce. You want the boost to be constant and stabilised. Also if you think of changing the airfilter do not make any big holes on the box because these engines have a know problem of burning the air-flow needle to much easily. And i can tell you that this thing costs....
Hope i have been of any help and not confused you any more...