I had the 3 MPS 2nd gen and have driven the 6 MPS.
My dad and brother both a 330i's
Although the 3 is slightly more rewarding to drive....
I would go for the 6MPS as its a great car, ultra reliable and something a bit different, decent 4 wheel drive system and a really great engine, once mapped and a decent intake. Like others have said it might be slightly more difficult to sell but i wouldn't let that put you off.
My gen 2, which is known to be harder to map than the gen 1 and 6MPS was putting out about 280+BHP with a map and intake. It ran about 5.8 0-60MPH from a front wheel drive isnt too bad. My average MPG over about 8k miles was 22MPG and it was driven hard most of the time + a trip to the 'ring
Good forums too, with loads of knowledge and tips for the MPS