I haven't finished TW2 yet - I played TW1 off and on for the first part of the year, finished it about a month or so ago, and have been playing TW2 for a couple weeks now. I would say that if you compare the writing of individual missions and characters, The Witcher franchise might come out a little bit ahead of the Mass Effect franchise.
However, I don't find the overall setup and premise of The Witcher as compelling. Mass Effect is detailed, but it allows you to grasp the general picture - multispecies galactic society with humans as relative newcomers - quickly and clearly. With The Witcher, I sometimes feel like I'm making decisions without really understanding what's at stake. The first game is set in motion by the Salamandra attack on the Witchers, but I didn't feel like I had a clear grasp on who exactly the Salamandra were or why their theft of the Witchers' materials was so important as to be worth everything Geralt goes through in order to track them down. (That, and I just generally prefer near-future sci-fi settings to vaguely medieval sword-and-sorcery fantasy settings.)
A good example is when Triss suggests to Geralt in TW2 that they just leave Flotsam and stop working with Roche. Do they have other obligations and affiliations that would either be solidified or weakened if they leave? Are there conflicts that might require their immediate attention besides looking for the kingslayer? Will they have a better chance at resolving those conflicts depending on whether they intend to leave or stay? And where are those other Witchers that they were with at the start of TW1, and why did they never reconnect with them after completing the Salamandra investigation?
The other difference in ME's favor is that, when presented with a grim situation and a series of bad choices, I like to be allowed to at least *try* to argue for a different approach the way Shepard usually can, even if I sometimes fail. A more Witcher-esque take on the geth and quarians, for example, would probably require Shepard to choose a side in ME3 rather than allowing for peace under certain circumstances.
Some probably see the more ambiguous aspects of The Witcher as being more realistic, but my own view is that fiction shouldn't just reflect the world back at us but should have room for considering better alternatives. I don't really believe that, when presented with a potentially violent conflict between two deeply flawed factions, all you can do is (a) side with one of them and hope to tinker around the edges by occasionally urging the leader to show restraint, or (

throw up your hands and let them kill each other. And yet those seem to be pretty much the extent of Geralt's eventual choices in the Order/Scoi'atel war in TW1.
(Of course, Geralt doesn't have the same stature that Shepard does, so he wouldn't be the best choice to attempt to negotiate a reconciliation anyway, but that's part of what makes playing him a little less rewarding.)