I played it thru the first time on Anti-Registration. I am going to go back in and blow it out on Pro-registration with the difficulty maxed out.
Pros
- simpler powers (only 4 for each person, not 663 or whatever)
- easy to upgrade powers or just ignore the system ( I did this and had fun)
- art work looks realistic
- story is more interactive and allows more user control. Not just the same thing
- Fusion (combo powers) are fairly easy to execute
- change team at virtually any moment after the beginning
- some of the battles make you think; not entirely just button mashing

Cons
- some may not like the character design/artwork
- I played through the game all the way and there are still unselectable characters (grrr)
- seemed to be a lot shorter (Maybe I just had too much time on my hands, but I beat it in about a week of moderate play. The other game seemed to last considerably longer)
- Some of the character selections are not the best. They could have kept Green Goblin, Songbird, that creepy Penance/Speedball/Soul Calibur rip off Voldo, and a few other people.
- even though I admit to exploiting it, some characters are fairly overpowered. Storm, for example, is immune to so many types of dmg being tossed around and resistant to many others. If you just hover around, and don't enter melee, you can toss lightning down and dominate. I suppose balance was more thematic and in-line with the comic book, but that left a lot of characters on the weak side.

My teams basically consisted of: Storm (see above), Deadpool (great dmg, Healing factor, teleport), Wolverine (see Deadpool+ strength), and either Luke Cage (cool voice over, taunt and God Mode power) or Juggernaut (cheapest stampede power action in the game. Pretty much was unstoppable when he was stampeding).

I might try online mode for kicks, but overall I enjoyed soloing it.