Personally, im a big fan of Halo 3.
BUT I think its wrong to compare the 2.
Your either going to be a First Person Shooter Fan
or
A RPG fan. If your a RPG fan, your going to like Mass Effect, and if your into FPS, then your going to like Halo 3.
I personally have a likeness for both types of games. Halo 3 is awesome but heres my personal opinion on M.E.
Not enough action,
Kinda Easy, just gotta learn how to play patiently,
No online play gets kinda boring
It is what it is, a RPG, and it does a good job at it, its not a bad game, just different from halo.
M.E. is only 1 of 3 games release, and its a GREAT story, i beat the game already and waiting for the continueing.
Halo 3 is 3 of 3. You may want to get both, Halo 3 first, and then Mass Effect next to start a new series of games to play
