I find the Britax a little easier to use on a day-to-day basis, in terms of adjusting the straps. The Diono is narrower by a LOT, if you're trying to ever fit them in something other than a giant SUV or minivan (or with more kids). The Diono is also taller, so I replaced my Britax with a Diono when my DS outgrew the Marathon at 3 something. (We're tall. He's tall.) The Diono is HEAVY because it has a steel frame, but this means you can do some things with installation that you can't with most seats (like when rear-facing, it can touch the seats in front of it. The shoulder straps can be below the shoulders when FFing, etc/)
We went with the dinos. Two main things - the taller shell so we can do extended rear facing for longer (I'd rather keep them in the safer position as long as possible) and because they are narrower, so that we can still fit a (skinny) person between them in the backseat. Our vehicle only seats five and we have already used the middle seat a few times to get a fifth person in the car at times when it was needed.
Re: Convertible car seats..
i have neither of those...we have Graco MyRide 65s....the boys have been in them since coming home from the hospital at 5 and 6 pounds...LOVE them!
Do you plan on using them sooner or later? If later I would not buy them now...I would wait till you needed them since car seats do expire.