Only seen 3 or 4 MPW (7 seat) 500Ls on the road so probably not enough out there to judge this yet. However, a huge number of 5 seat 500Ls are 1.3 Multijet Dualogics and not seen any broken down, appear on here as having problems etc.
I drove a 1.3 Multijet 5 speed manual MPW and whilst it's no ball of fire, it certainly felt safe & competent on the 60mph journeys between Oxford & Cheltenham I used it for.
Personally I'd have the 1.6 Multijet if I planned to carry 7 people or luggage regularly, but the 1.3 is a fair bit quieter and gives off less vibrations in the cabin compared to the 1.6. Of course if you need an auto it has to be the 1.3 as that's the only auto available.