(note: only the Cappuccino color is $199- be sure you select that color!)
I have both the Marathon and Decathlon and they are both great. Just a few differences between them.
Marathon---The most basic model of the Britax seats. It harnesses to 65 lbs, rf 5 to 33 lbs. It has a rf tether, ff and rf lockoffs. Has the lever style harness adjuster, 4 harness heights, and the crotch strap does not adjust. It is very padded and comfy just like all the britax seats, and has about a million cover choices. Will last till the child is ready for a booster most likely. Good for children that need to rf still (under 33 lbs).
Decathlon- Harnesses to 65 lbs, rf 5 to 33 lbs. Has rf tether, ff and rf lockoffs. The ff lockoffs may damage seatbelts. It has a push button style harness adjuster, 4 harness heights (rumor is they may be a little taller than the ma/bv), and a three position crotch strap that gives 1 more inch of room as compared to the marathon and boulevard. It has infant inserts so is great for a small baby. It also has a removeable toddler head pillow. It has more limited cover choices than a marathon. Will last till child is most likely ready for a booster. Good for very small babies because of the infant insert but a ma/bv may be a better choice for an older child that won't need the infant insert.
These are fabulous deals!