Mebane is good, but there's no chance of him getting any better (his % of snaps has been decreasing over the last three years anyway), and I think a 6 million cap hit is a little high for a player who is used as a situational run stopper. I mean he's good at that for sure and adds some bull rush, but there are a lot of guys (perhaps not quite as good, but still serviceable) who can fill that role at a much, much lower cap hit.
FWIW though in terms of salary cap management between these players they still would go Mebane over Maxwell, IMO. The cap problems aren't for next year, they're for the year after that and the year after that. As a result Mebane with one year left on is deal is actually a lot more manageable and "friendly" from a cap perspective than a re-sign of Maxwell who will have real money kicking in two or three years down the line (when all the real money for everyone else -- wilson, wagner, bennett, thomas, sherman, etc.) will have totally kicked in.