I have both, and they are hands down my two favorite folders. It's tough to go wrong.
The PM's clean construction is better suited for hard, sustained outdoor use. Fewer moving parts, grippy G10 (the PM has MUCH better handle security, particularly with wet hands or gloves), thicker blade, and lighter weight. The knife is exceptionally easy to clean and maintain in the field.
The 710HS with M2 steel is an outstanding cutter -- blade shape is awesome for certain tasks. But my 710 developed some vertical blade play after I used it for basic light camp chores on a 4-day mountain climb. While I felt there was no risk that the axis lock would disengage, the blade play did bother me, and I can't seem to fix it myself.
The PM's compression lock, on the other hand, has never given me problems. On the one occasion that I got some vertical blade play, I simply rotated the eccentric pin a bit, and the lock up was tight again.
As far as looks, the 710 wins hands down, IMO.
As hard as it is to say, if I had to go with one, I'd take the PM.