H.B.
Ah the downfall of the flowery seat covers... :-) I was so tempted by them as well, but bought my Britax seats on sale so ended up with neutral covers. Thank goodness, as I've done just as you are - each child moves from their infant seat to the Britax Roundabout (now containing my petite 2.5 yr old daughter), then the next larger seat I got is the Britax Marathon (my tall 40 lb 4 yr old boy is in that now, fitting quite nicely with room to grow). I'm debating right now whether to get a second Marathon or a booster style seat for him when my 6 month old baby boy is ready for the Roundabout (within the next few months).
If you like Britax, I would just stick with it and get the bigger seat. Personally I love mine. I've found great deals at this website http://www.albeebaby.com - often 20% or more off the store prices, and free shipping.