They both sucked as games, but I wouldn't call that a ripoff in the same way you're talking about it.
Excuse me, I'll have you know that Mass Effect is an awesome series. And Halo's not so bad, but it's not in my top bunch of games.
As for the topic at hand, I wouldn't say ME rips off Halo in any way.
The Halo rings do their killingness to starve the flood, the Reapers kill things off because they're omnicidal machine gods (unless you want to get into the ending. I do not want to get into the ending).
The flood aren't machines, and they tend to act in a more bestial manner than any of the ME synthetic life. The stuff in ME is more the "conventional" type, AI that's got free will.
Geth (Heretics, anyway) believed Sovereign was their god. Saren was indoctrinated. Basically a union of mind-control rather than mutual hate and coercion.
Again, Saren only "thinks" that life would get spared if he sided with the Reapers because he was too far gone in terms of indoctrination to even consider otherwise.
They are two very different games with two very different settings. Because they're both set in space, saving the world isn't a very special thing in a galaxy filled with billions of stars. You have to up the ante, which invariably means saving the galaxy and all life in it.