Here's a jumbled response:
The Boulevard has True Side Impact Protection (headwings) while the others don't. The Decathlon comes with infant padding, which is nice, but there have been a lot of issues with the harness adjuster making many return it for a Marathon. The M lasts longer than the B by height. The B has issues with head slump as the headwings push the child's head forward. The M and B have LATCH bars, while the D does not. The M and B post March 2008 RF to 35 lbs, the D only goes to 33 lbs. All three seats are very similar in size and shape. The M has the most choices for covers. The B has an infinite harness adjust, the M and D you have to rethread.