I don't have a different suggestion for the storage, we are using the same thing at our house but haven't had any problems.
As for the carseat. We had a boulevard and a round about. Both our kids are tall for their age, our daughter si 1 1/2 and 34", our son is 3 1/2 and is 43". The boulevard was clunkier than the marathon and the knobs were wearing on our seats so I sold the boulevard and kept the marathon for our daughter. Our son could have stayed in the boulevard a bit longer at age 2 1/2 but the buckle between the legs was to tight. We weren't ready for a booster for him either and needed him in a 5 point harness for a while longer but needed something that would accomodate his size better. We bought him the britax frontier. It has the 5pt harness and can be worn up to I think 85 lbs and it also converts to a booster for up to 100 lbs. At 43" he has a long way to go with the 5 pt harness for height and we still can move the buckle between the legs out another 2 inches (its moveable) as he continues to grow. It's heavy but it's been great and I like knowing that this will be his last seat based on the size of it. Our daughter, while tall, is petite (he's 44 lbs and she's 23 lbs), she should be able to get most of the 45 lb max out of the marathon. By then she should be able to have the frontier.
Check out your local babiesrus stores for clearance and/or save your 20% coupons they hand out. Regular price for the frontier is usually $279, I just saw one on clearance for $239. Plus you can get 20% off w/ the coupon. Their clearance is just to get rid of floor models. They don't resell returned car seats and they have all of the pieces and manuals.