Out of those I'd prefer the diesel. I wouldn't worry too much about the DPF but do check the timing chain. Start it from cold and listen for a loud rattle for the first 10-15 seconds then rev it up a little to about 1500rpm and check it doesn't sound like a box of bolts from the back of the engine.

Also check for oil leaks around the valve cover and oil filter housing (neither the end of the world).

I run a 2008 Volvo V70 and it's been a great car. Any used car is a bit of a gamble but my car does all sorts of journeys with little complaint.

The 320i is likely to be less problematic than the 320d but won't be as interesting to drive

But, it will come down to servicing and how the car has been driven. If the diesel has done mainly long runs and has had regular oil changes then the chances of EGR, inlet manifold, DPF and chain issues will be minimised. Those are the main areas of concern for BMW diesels.

The petrol engines can damage injectors and coil packs but these should have been sorted via TSBs by BMW dealers. You can always ring a dealer and ask if there are any outstanding TSBs that either car is subject to - basically they will rectify for free. TSBs are not recalls.

As you're not going to keep it, buy on condition/provable service history otherwise you'll end up spending e.g both cars will be due a gearbox service (they're not sealed for life)

I had a 2008 E92 320i for 4 years and 60k miles and the only major failure was the differential which would have been fine if I'd known to get the oil changed on it (not an expensive job)

HTH