I LOVE my Decathlon. My DD is 8 months old and big (about 30" and 20 lbs).
The reasons I went with the Decathlon (Instead of Roundabout or Marathon)were:
-It is the only Britax with 3 different positions for the crotch strap.
-She can stay rear facing in a 5 point harness to 35 lbs (same as Marathon)
-I intended to keep her in her infant seat for as long as possible, but she outgrew it at 6 or 7 months. The extra padding in the Decathlon is wonderful!
-Britax can be tethered rear facing
-It fits rear facing VERY well in my smallish backseat.
-The "Tiffany" fabric is soooo cute :)
The only negatives I have heard about the Decathlon is that it can be hard to tighten the straps when baby is rear facing. I have had no problems at all getting her nice and snug.
It is essentially the same seat as the Marathon. Same sized shell, just with a bunch of nice padding and the adjustable crotch strap. The padding IMHO is nice for babies on the smaller end of the seat's size range. My daughter loves that she can sleep without her head slumping to the side.