www.babyage.com www.albeebaby.com www.elitecarseats.com are a few that have good prices on Britax often.
The Marathon is the basic seat. It will rear face to 35lbs, forward face to 65lbs, and generally fits kids rear facing to age 2/3, and forward facing to age 6. We had one, it was by far the best car seat we ever owned! I loved ours.
The Boulevard has side impact protection and infinite adjust. Meaning you rethreading of the harness to adjust the height, you just turn a dial on the side. THe top harness slot is slightly lower than that on the MArathon, so you will lose time height wise on a Boulevard. Its a good seat though.
If you are looking to save some money, look for the Roundabout 50. Its the same shell as the Marathon, but only harnesses to 50lbs forward facing, and has no belt lockoffs. My boys are BIg boys, but my oldest didn't hit 50lbs until he was 5, and my youngest didn't hit 50lbs yet and he's almost 5. The seat is a great buy for a Britax seat because you get all the height of the Marathon for about $100 cheaper.
Do you ahve a Britax preference? Becuase there are so many seats out right now that you could purchase that would rear face longer and forward face just as long for less money. The BIGGEST bonus of a Britax is the ability to tether while rear facing. The rear facing tether option is HUGe because it reduces rebound dramatically in a crash.
Graco Myride 65, First years Truefit, Evenflo Triumph Advance, are a couple other GREAT convertible seats if you are still looking for a seat.