I came from GTI to a 128i (with a slight detour to a 328xi) and can easily say, in term of pure driving joy and fun, 128i is the GTI's rear wheel drive brother. You will not miss your GTI and will have even more added fun with RWD.
I had a Mk5 GTI which didn't have the greatest interior, but I find the 1's interior to be the same if not slighly better. In your case you have a killer interior in the Mk6, so the 1 may be a step down for you.
I was in the exact same situation as you, looking to buy and not wanting to deal with long term issues with the 135. I test drove both and although the 135 is a beast, it definitely wasn't as fun to drive as a 128. Even taking away the reliability issue, I'm still pretty sure I would have gone for a 128. Sports and 6MT are an absolute must. Good luck!