Given that choice go for the AVGAS every time.
As above, I'm sure they will have more AVGAS burning aircraft, and those diesel converts are just terrible. They take an aircraft which used to have a smaller lighter AVGAS engine, then fit the large heavy diesel one with a gear box, two fadec systems and a computer controlled VP prop. This means that there is more complication, which you just don't need on something being maintained by the local spanner boy being loosely overseen by someone with an authorisation, and it's a heavier engine on smaller engine mounts, which only buckle when you have a nose heavy landing, which you almost certainly will at some point during training.
If you must fly diesel, find something where the aircraft was designed to accommodate the engine, like a diamond. Still more complicated than it needs to be for GA maintenance.
Also, never pay up front.