I think 103 is a better motor, not necessarily than the 104 but better than the 3.0 liter 104, which is not up to the standards of the 3.2 104. The 3.0 liter 104 motor was Mercedes first mass produced 4 valve motor and they rushed it to market trying to keep up with the Japanese. The 3.2 in 1993 (or 92 in the S-class) has been sorted out a little better. But there's a lot of people who like their 3.0 liter 104s - and it is a little easier to work on than a 3.2. But neither one is as easy to work on as a 103. The 103 valve seals do not require removal of the head. I think it's about a 4 hour job, so not a big deal.

To me they are both good DIY cars. I would go with whichever one you like better - 2 door or 4 door - or whichever one you find a nicer car. Also, I think cars with ASR are to be avoided - it does not age well.