congrats, man
Would be tough to sit any of those RBs for Mason but Mason at home vs the Raiders is a pretty good play - Raiders have a better pass than run D, so Rams will likely go run heavy in this one.
Lacy should find it difficult to run on the Pats and will likely be TD dependent. He hasn't been running well this year, unless he is playing Minnesota, and is heavily dependent on a big receiving play or two, which he may find tough to come by vs the Pats (they aren't as bad vs the run as the media would like you to believe).
Lynch is having some injury concerns and above all, his center, Max Unger, is out. 49ers run D isn't too bad but isn't immovable either - Morris had a great game @49ers just this weekend - but it's the absence of Unger that makes Lynch's production questionable for me. Just for reference, Unger went down vs the Redskins earlier this year and missed the following 3 games vs very beatable run Ds; Lynch was poor in those exact 3 games and had a great day in Unger's comeback game. Coincidence? I think not. At the same time, he may well be Seattle's main weapon vs the 49ers, so he should see a lot of the ball.
It's a tough call who to sit between Lynch and Lacy but I'd try to find a way to start Mason this week.