The different?.

The new characters are very cool, on that merit alone it is worth it.
The DLC from the previous (sans Shadow Battles) carry over and with a bonus for Jill Valentine and Shuma Gorath. There are way more costumes (6) available for all of the other characters.

There are new challenges that you can get titles for.
Galactus mode is pretty cool, (hold L1, Select, and push X while the "cursor" or "selection" is on "Arcade")
That is unlocked from the beginning if you had a save file from the original, or after you earn 30,000PP

They tuned the characters in a dramatic way. It is done more realistically than in Original. There is a profile section unlocked already (mine was, anyway) and it shows you the stats of each character, there is model mode, and color change right from that menu.

The Stats in the Bio are actually (I'm pretty sure, being a comic collector for 21 years now) based off of Marvel's own Power Level System; the thing is, though, that the stats REALLY matter and make a huge difference. If someone has low stamina and they are again, let's say Hulk, they low stamina character will go down quick.

The power levels on the original, for me, were tuned too balanced. This one is more realistic (well, in a fantasy world "realistic", anyway) and when they tuned the stats some characters are really $crewed over. They tuned this one to be less balanced, which I think is good, poor Phoenix Wright, he's basically the "Dan" (Street Fighter Reference) of UMvC3.

It is way more challenging in that aspect, as if you choose the characters you loved your whole comic and/or gaming life and they are not really that strong compared to the others in UMvC3 (Arthur Vs. Iron Man, for instance, pretty much means you have to play harder if you want to be Arhur; that was just an expample.)

None of the characters are invincible, though, any character can win against another, it is just more (or less, depending on your choices) difficult to beat.

Another thing I like is that you have to do actual button/stick moves (Hyper Moves/Combos) to actually pull off a "Team Combo" or a "Hyper Combo" instead of just pressing two of the buttons on the top of the control.

This version is much more than just an update/upgrade, it really should be MvC4 (But it they did that I WOULD want many more features, even with how sweet this game is)

So MvC3:FoTW, I give a 4.0/5 and UMvC3 I give a 4.5/5; the only reason it is not a 5/5 is because the story mode is still pretty lame. Just beat it with Captain America, you'll see what I mean.