Buy the H. Stuck engines can end up costing a fortune! If you look at some postings about a 450 diesel about a week ago, a guy bought one of those in perfect running condition and has spent a fortune on the engine anyways - found a cracked head, block, etc. It really depends on why the M is stuck, did it sit outside in the rain for 3 years? Did it spin a rod bearing or run out of oil?
In any case, buy a running tractor over a stuck one. My vote goes for the H.