Cathy, I've had several different car seats and prefer the types like the snoozer or pet gear over the solvit type. The snoozer and pet gear seats are strapped in with the seat belt, utilizing the car's own seat surface for stability. The solvit types are just kind of hanging in the air strapped in around the back portion of the car's seat and head rest. I didn't feel as though my kids were as secure in the solvit type as I do in the pet gear or snoozer types.
Also, my dogs are small and in the solvit type seats they tend to sink down inside the bed area, and can't see as well as on the little raised bed area of the snoozer and pet gear types.
I thought my pet gear seat was a medium but after recently ordering a medium off the CSN stores clearance area and getting it home, I now think my other is a large size (bought on Overstock.com). The larger size is plenty big enough for Ruby and Reno, 3 and 4-1/2lbs. I bought the new one because now we normally travel with three
Both seats fit just fine in our smaller size SUV's back seat.
I wanted to add, having ours ride in car seats does seem to help with car sickness. Also, just now saw the size of your kids....maybe two small size pet gear or snoozer type seats would work better?