Since your daughter is so small, I suspect that she may be able to get the full use out of her
Marathon. In fact, she could even still be
rear facing.
If you move your baby, the
Marathon will expire before she's outgrown it...and you'll still need to buy another seat for that child.
You could move your 2.5 yr old into a
Nautilus...but it SWALLOWS small children--I've seen pics of a 30 pound 3 yr old---and while she technically fit---she was just so tiny in the seat.
Just some things to consider.
Susan