That picture is going to help a lot. Just in case: the blue knife on the bottom is the M390 PM2 (the top knife is a Military), the third from the bottom is the 710. You can see that they have roughly the same length handles, but due to the curvature and blade shape the PM2 will take up more room in your pocket width-wise. The picture also shows how much more blade and edge length you get with the 710.
As far as the comment about the stud on the 710, my experience with Axis locks is that I open the lock and flip them open, so that may or may not be an issue for you. They are both great knives with huge followings. For me, if it was a choice between the 710 with M390 and the standard PM2 with S30V, I would take the 710. If it was both knives in M390, I'd probably be here asking the same question you are!
Hopefully the picture helps and one just appeals to you more. It sucks trying to pick a knife without handling it (support your local B&M - I buy my cheaper knives there). As has been said, if you can afford it, buy both and sell the one you don't like. Personally, in this case, I think if you buy both, you'll keep both!