If you've actually narrowed it down to just two cars, I would just go try them out.
Are you planning on more kids? How close is your 7 year old to not needing a booster? I think the buckling in in most seats w/three carseats across is a huge PITA (even if they fit, it's hard to get the booster buckled) but it might be easier since I assume at least one of your Marathon's is still rear facing.
Are you planning on more kids? How close is your 7 year old to not needing a booster? I think the buckling in in most seats w/three carseats across is a huge PITA (even if they fit, it's hard to get the booster buckled) but it might be easier since I assume at least one of your Marathon's is still rear facing.