Converting MLV to Prores
I'd pick something that looks flat and unsaturated.  Those are usually the ones designed to retain detail.  If you see clog or slog2, those are good.  Rec 709 will throw out info, so don't use that.


  What should inform this decision?


Trial and error.  A lut is really just a starting point for your grade.  Try some different ones out.  If it absolutely butchers the color, then it's probably not a good place to start.  If you see clipping or banding, then you've chosen a lut which was meant for a different situation.  As you try each one, look at the trace on a waveform monitor and vector scope.  If anything goes out of bounds, pick something else.  Do you notice any kind of granularity, banding, or stair-stepping in the traces?  Look for one that's more even.