The Dolphins' offense is better, but their defense is worse. I don't know if the addition of Marshall will offset that. He will definitely make Henne better, but I don't think Marshall is the difference between making them a 7-9 team last year to a division winner this year.
Here is Miami's starting defense right now (as best I can figure it out):
DE: Randy Starks
NT: Tony McDaniels
DE: Kendall Langford
OLB: Cameron Wake
ILB: Dansby
ILB: Crowder
OLB: Quentin Moses
CB: Vonte Davis
FS: Chris Clemons
SS: Yerimiah Bell
CB: Sean Smith
Close but couple of changes.
DE Starks had a great year and was close to a pro bowler (many thought he was robbed).
NT Soliai - Young player with potential played pretty well at times when Fergy went down.
DE Langford - solid player
OLB Wake was Miami's best pass rusher if you measure it by sacks or pressures per snaps played, not sure about the rest of his game.
ILB Dansby - Best ILB Miami will have and HUGE upgrade over Adolye
ILB Crowder - Solid especially with someone better than him next to him
OLB Moses/Anderson - I Agree UGH. Anderson has shown flashes but nothing consistently.
CB Davis - 2 picks of Brady in 2 games is a good start
CB Smith - Solid young corner
FS Culver - I think Ty Culver would start over Clemons but still a huge question mark. However if you watched Gibril Wilson last year you'd realize that in coverage it can't get much worse.
SS Bell Pro bowl safety, excellent in run support
Not as bad as you think. I'd feel better with a new OLB, FS and NT. It wouldn't surprise me to see JT come back despite all this Jets talk.
The Dolphins and Jets look like they've improved but as long as NE has Brady and Belechick they're the team to beat in my eyes.