Difficult one! Most people will have one or the other and will probably favour the lathe they have.
I have an ML10 in good order and with sufficient bits and pieces to keep me happy. I suspect one of the real difficulties will be in the cost of accessories for an ML10. Just look at the asking prices for fixed and travelling steadies. Sit down before you do! having said that, such things are relatively straightforward to make. but do you want to make bits for your ML10 or make projects like locos?
Mini- lathes are getting to be very good in quality and spares are good and plentiful, not to mention inexpensive. ML10s command better prices second hand, in good nick. All a bit of a minefield to choose between the two.
I will stick to my ML10, although I suspect the overwhelming majority would say keep the mini lathe.
Andrew.