Should I play Mass Effect 2 or The Witcher 2 first??

I really want to try these games out as I've missed out on them these few years. I never got around to finishing Mass Effect 1 on my 360, but I would guess I was about 80% through. I have never played Witcher 1. Would I be missing out on a lot of critical story points?

Anywho, which is more worth the time?