I agree with TD. The three most important apects of an sports car or hot hatch are performance, steering feel and driver enjoyment.
The M135i trumps the RS3 on 2 of these and does this for over 10k less!! Look how unhappy CH was in that review, he said he didnt care if the BMW was slower than the RS3 because irrelevant of its time, it was an unpleasent chassis and whilst it might have been quikcer than the M135i on a lap, it was still slower on the drag race, despite having to the advantage of the laucnh control and traction, so give it a rolling start, i reckon it would be beaten by a larger margin. Its going to be embarrassed bigstyle when the 1M is released.
On the subject of the new S3, i highly doubt it will be in the same league as the RS3 performance wise, and no doubt handling as well, so the difference between the M135i I expect will be huge. Especially considering the S3/M135i are direct competitors.
Previously VAG had the trump card on tunability over the BMW range, now i dont think they have that anymore.
Some people get hung up on VAG being superior to everything else on the market. There is life outside of VAG
Look at how much money we all throw at VAGs to make them handle well, with the BMW, you dont need to waste that money.