I think everyone knew the Bills would be a losing team last year and RBs on losing teams typically disappoint so McGahee's fall was not a huge shock.
If you believe Miami will be a winning team this season then don't hesitate on Brown.
Personally I think they are probably a .500 team but with a few lucky bounces they could win 10-11 games. So Brown should be a safe, though probably unspectacular, pick. On a good team I see him as a Rudi Johnson type runner with better hands, but why take Brown when you know that Rudi will give you 1,500 yards and 12 scores?
Curious to know why you think Miami is a .500 team this year.
Back in the day, i.e. 2-3 years ago, a player took 18 months to come back from an ACL injury now Daunte is coming back from ACL, MCL AND PCL replacement surgery in 10 months. He is in a relatively new system, though Saban says he is keeping elements of Linehan's system there is a different guy (Mularkey) calling the plays. I do not think Joey Harrington, learning a completely new system to him, is the answer if Daunte can't last the year. The offensive line is average at best. They have one good WR and Randy McMichael has never done anything past week 6 of any season. They also have nothing behind Ronnie Brown, who is still an unknown commodity when it comes to what kind of workload he can handle. The defense was solid but is another year older, plus Jason Taylor is nursing a back problem (and who knows what kind of impact his divorce from Zach Thomas' sister will have in the locker room)/Saban appears to be a solid coach but I am not convinced after one season. The six game win streak at the end of the season was nice but it doesn't mean squat 9 months later. There have been plenty of six game stretches of high productivity at the end of a season that have not materialized into anything the following year (Buffalo two years ago is a great example and on an individual level Kevin Jones and William Green come to mind).
Like I said, if they get a few lucky bounces they could be an 11 win team but I see a razor thin margin for error with this team, i.e. lots of games decided by 7 points or less.