lets see... i own HERO5, MnM, MARVEL saga, MARVEL the new thang, and even the counter collection 16 page jobber.
I have GMed or played in MnM and HERO5.
I love the XMEN.
I would recommend MnM.
The new Marvel thing even used the xmen and brotherhood as examples of their combat system in the book, but it felt like no Xmen battle i had ever seen (and did not look like fun as an RPG session eventeither.) If you think Jean and Scott vs Mystique and Sabretooth should play out as jean shutting down both the bad guys automatically, one per round, while scott is either jelloed by sabertooth or just keeping sabertooth busy while jean finishes the fight... then Marvel is the way to go. (I am not kidding... those were the fight examples.)
With MnM for an Xman game, you need to not allow too many PCs with collossus level armor plating, some people mistake the fact that you can build a PC with colossus level plating to equate to all the PCs should have colossus level plating... and build most of the characters with as much "amazing save damage" or more than armor, protection and force field and you end up with characters who can be hurt but not easily KOed. if you go for the XMEN suits = armor days instead of the yellow spandex days, then most XSUITS can be armor between +3 and +5 and various levels of amazing save damage (combat training) or outright evasion (avoiding damage by dodging) can reflect the various levels of training and keep Scott up and running in a super fight as well as colossus while making scott worry about guns while colossus does not.
The MNM game i am running now is just a little higher than this, with more than half the NPCs supers at risk from guns.