Another vote for the Marathon/Wizard here. We bought DD a Roundabout and it suited us well for a couple years, but she's almost outgrown the height at 2 1/2 yrs and is still only about 29lbs, so weight is not an issue. We just bought her a Marathon so we can pass the RA to the new babe due in Feb.
If you are planning another child fairly close in age then the RA might work for you as it did us (vs 2 marathons). But since the seats "expire" in 6 yrs, you can't hold onto to it forever.