Look at the Wikipedia entry for all the differences between the many versions of MUA.
The PS2 got the basic version.
PS3, 360 and Wii got the next-gen version with Colossus and Moon Knight.
PSP got other extra characters etc etc.
360 got DLC downloads that eventually ended up in a Gold edition that totalled 8 more characters than the PS3 and Wii, so 10 more than the PS2.
This time around it looks like there will be 24 starting characters, which will hopefully be 24 for the PS2 and more for the others.
Juggernaut has been confirmed as PS3/360 only and pre-order only and possibly North America only.
Wii owners "will get something special" but no details yet on what.
DLC, other exclusives etc are all speculation at this point and will depend on how much money the platform companies will throw at Activision and what other sweetheart deals will be made.
Originally it was going to be "Marvel Universe Online" and exclusive to 360 like DC Universe Online is exclusive to PS3, but the Marvel project was canned. If DC remains Sony exclusive it is a logical guess that Microsoft will invest in the 360 version of this game being more attractive than the PS3 version.