I don't like trying to sell people on things, so I don't care what you choose honestly. However, The Witcher is the better overall series, since you asked for thoughts on the matter. Unless you just have to shoot a plasma gun. Then I suppose Mass Effect has that battle.
Right. I could go into a long winded discussion, point for point, but this is a highly subjective topic. I think Mass Effect, and Bioware in general, are paper thin games with bland level design, silly dialog, weak RPG mechanics, etc. That's about the long and short of it.
I think the more interesting topic is that Bioware has degraded as a development company, falling into the same "simplify everything" ideal that Bethesda has. IMO, they haven't made a great game since Mass Effect 1 and Dragon Age 1.