That's definitely an interesting way to do it. I think I like it... but then, my favorite team is the Steelers.
So there are 10 teams taken (you and 9 others), which means that there are 18-27 viable players that you cannot
play (2-3 WRs per each of the 9 other teams). Having to choose 3 quality starting WRs is sometimes difficult enough when you can trade and draft from all 32 teams.
...Argh. I misread that as you have Ryan Matthews, not Williams. Had to delete my post.
I think this all hinges on how comfortable you feel with Charles and Beanie as your backfield, with Williams as Beanie's handcuff. You've got two guys coming off injury, though you have the backup player for each.
What do you feel more comfortable with? A combo of Foster with Williams, Best, or Hillis (if Charles doesn't recover), and a thin WR starting crew; or Megatron, with good depth behind him, and possibly a backfield of Best, Williams, and Hillis (if both Beanie and Charles don't recover well)?
Personally, I would take the trade. That way I absolutely know my WRs are set going into the season, and I only have to hope/pray for one of Charles/Beanie to be healthy (you can WW a hot RB during the season). If you don't take the deal, your best case scenario is having two #1 backs, and not too much at WR. If you make the trade, your worst case scenario is a backfield with Hillis as the workhorse, but solid WR contributions; with an upside of having two #1 backs, plus great WRs.