Hi! I'm sure someone with a better feel for your specific vehicle will be along soon, but in the meantime I wanted to ask whether your DD is tall for her age/weight or more average/short. The MyRide and the Marathon are about the same for stats, but tend to have a shorter shell than some other options like the Radian and a couple of seats not on your list, the Complete Air and the TrueFit. For a lighter but taller child, a seat with a tall shell is a better choice.
In general, the Marathon does not get recommended around here right now because it has a low, 30-lb limit for RF, and that combined with lower slots/shorter shell, less leg room for RF (not a real issue but why not get more) and a higher pricetag makes it a less desirable choice. Britax is putting out new seats sometime this fall which should be better in terms of weight limits but the current models are not as good for ERF. If your child happens to be petite, though, she may have plenty of room to RF in this seat to a good FF age (ideally 3-4yo) if you feel you love it after considering the options.
Another consideration if you plan on having more children to pass seats down to may be the life to expiry for the seats. The MR and MA are both 6 years. The Radian and CA are both 8 years, and the TF is 7 years.
It doesn't matter which side of the van the seat is on in terms of safety, but it may make a difference in convenience and fit.
- Are there any other kids in the van? Where do they sit? What age/wt and what do they ride in (and are you able to pass down an older seat and upgrade the older child instead)?
- Which side fo you load from? I like being able to access all the kids from curb side - I unbuckle from driver's side then have everyone climb out the pass side.
- If you need more room for the driver's seat to go back, put the seat in the pass side.
- If you have advanced air bags and need to be able to touch/brace a Radian to make it fit, you may need to put it on the driver's side which usually doesn't have the same sensors that make the pass side unable to brace.
Hope that helps!