Welcome.
First, the snowsuit in the car is dangerous. It will compress in a collision. You mention it's hard to do up her straps. Have you put her in without the snowsuit but the straps in the snowsuit position? That's how much air there would be in a collision, and having her in with her snowsuit basically means the harness is that loose on her.
Given that, she's probably got a bunch of time in the Marathon. Then you can decide on the Frontier, Parkway, or Marathon. Though the Parkway has been discontinued so the availability may be less.
Copied from
http://www.elitecarseats.com/Britax-Monarch.pro:
"The Monarch Has:
* Expanding padded armrests
* Same weight, height, and weight capacity
* Option to be a backless booster
* More side impact protection for torso
o The Parkway loses some of its side impact protection when the headrest is raised, while to Monarch has high side-wings that protect all the way to the shoulder
The comfort factor on the Monarch is significant as well; with a triple layer of padding in the seat area and easily removed, machine washable plush fabrics. The headrest itself is adjustable with just one hand, and the seat can accommodate children from 30-60? or 30-100lbs. The padded armrests expand away from each other, creating space for bigger passengers that may need a wider seating area. There are also retractable cupholders (which the kids love) are in place on either side of the base. The Monarch is just the latest addition to the Britax family, and it proves once again why Britax is at the upper echelon of car seat comfort and safety."
Wendy