Tough decision!, eventhough that MSW has a better matchup than Megatron's, Seattle is still an average pass D, they rank 15th in the NFL, but they are battling major injuries, so they are getting worse.
So far, Megatron has shown us that he hasn't a lot of chemistry with his new QB, as MSW does have, MSW had a bad game last weekend, but I heard that he was running routes well and playing well overall, but it was Garrard's fault for MSW's underachievements. So right now, if you go with MSW you gotta hope that Garrard plays well for MSW to have a great day. In the other hand, if you go with Megatron, you gotta hope that he is totally over that injury, that Stafford plays well and that the chemistry will be there. Moreover, Megatron will be likely double covered in most of the plays, thing that won't happen with MSW, because they have to put an extra man in the box for MJD and you can't leave Torry Holt alone, because he will burn you.
So my choice would be MSW just because he is more of a secure option, but if Megatron explodes, you could be missing 20+ points.
Hope it helps!