I say Fallout because it's lore is much more expansive and in depth. The team would be allowed to focus on story and game design instead of world building and lore.
Mass Effect is too small right now and there would be a hell of a lot of work expanding the lore. For example, the comics and books all keep revisiting the same damn locations, mainly Omega and the Citadel. The second game introduced a lot of new locations, but the third game revisited too much old stuff and introduced very little new. Even the new stuff you were there for a very, very short period of time and not really allowed to explore.
Hell, they barely started to introduce females of other species, we've only seen one child the entire god damn series and that was a human. That's how damn small the Mass Effect universe is! They've been missing a huge chunk of every species that exists. Up until ME3 the Bioware team had no idea what female Salarians, Turians, or Krogan would look like. Considering two of these three were wearing space burkas for no explained reasons(what is lore?), I'm willing to bet they still haven't figured out how the females are suppose to look like cause their design team's minds are too wrapped around tits and ass defining what is female.
The only reason I'd vote Mass Effect is for the chance for a team with a brain, outside that Bioware team in Canada, to figure out how to fill in all the massive, gaping holes in the ME Universe that over the course of three games, a comic book series, and four books, Bioware couldn't figure out.