The
Roundabout will be outgrown around 3---so you'll need another harnessed seat, plus with it's short shell, you won't be able to get the max amount of
RF time.
The Decathalon---lots of people have problems with the harness adjuster---and you're paying extra money for the inserts.
I don't think you could go wrong with either the
Marathon or the
BLVD. Both are good seats that will likely last your child to 5 or 6...and by then...the child will likely be ready for a
booster(both in maturity and size).
Susan