The maxi-cosi is much shorter than the Marathon. But I believe I have seen posts on here that people have been able to get a marathon rearfacing in the same car as yours. The Marathon really would last much longer both rearfacing and frontfacing than the maxi-cosi.
I have a maxi-cosi and a marathon. I did not like the Roundabout at all (I tried it at the store). It is NOT like a "mini-marathon", it doesn't have all the same features. The maxi-cosi is nice, but it is almost as expensive as the marathon and is not quite as nice. Both seats are easy to use, but the marathon has lockoffs for both rear and front facing. I love the cover on my midnight owl priori (which is purple, not blue), it doesn't pick up dirt as much as my mariposa marathon. But the priori's harness, chest clip, and buckles are the exact same as is in the scenera so the buckles can be hard buckle.
If I were you, I would go to babiesrus or somewhere else that you could try a marathon and see how you like it/if it installs well in your car. Also search around on this forum to see if you can find any posts with people who have the same car as you and see what seat they have in their car. The marathon can be installed more upright for an older child than it has to be for a little baby, so that it takes up a lot less front to back space.
Also,
here is a link to a post that I made about the maxi cosi priori. It has pictures of it and the marathon.