Well gwilo I've just done what you're looking to do. I had a 1998 1.8t A3 with a Jabbasport generic remap on it and have just upgraded to a 2001 AMK S3 about 5 weeks ago, awesome car. The reason I went for a facelift was I was told by Jabbasport that the facelifted AMK & BAM's have better electronics with less sensor problems and it's easier to get more power out of them from a remap, don't know how true this is but I guessed they must know what their talking about. I specifically went for an AMK as I wanted the lower car tax bracket, I'd rather spend the money on mods than give it to the government. I think so far could have been a false economy as have been spending a fortune on fixing wear and tear items which I might have been able to avoid by buying a newer BAM.
I would say one thing about the prefacelift model vs the facelift's. Audi really dropped the quality of the interior finish on the facelifted S3's, the buttons with their rubberised finish are terrible for rubbing off, I'm spending hours searching ebay for replacement parts just for the buttons because the finish on mine has rubbed off. It's a real shame Audi don't do facia replacement kits for the stereo and the climiate control which is especially bad for marking up. My old 13 year old prefacelift A3 was immaculate compared to this S3, no issues on that with the finish rubbing off....what were Audi thinking when they designed the interior on the facelift. Whatever you do don't let anyone with long nails press the buttons! I do like the look of the facelift interior better but the ease at which the buttons scratch really is a shame! What was wrong with the older black harder wearing finish of the prefacelifts anyway!