Best new seat for my 2-1/2 and 4-1/2

[HKandKids]

My daughter is 37.5lb and 40" tall, and her seated shoulder height is 22".
My son is 32lb, 35-36" with seated shoulder height of 20".
She is currently in Marathon and he in Roundabout.

Both of their heads are within 1" of top of seat even though they are well within height and weight.

I am considering Nautilus for both but have heard good things about Recaro. I am very interested in keeping them harnessed as long as possible. What would be the best choice for them out of these OR OTHER?

I drive a Hyundai Elantra (small sedan) so they are close to the doors with no side curtain airbags.

[heckyeah]

I just bought a Britax Frontier and it allows my daughter to be harnessed until she's 80lbs and then becomes the booster.

Jen

[arivecchi]

I'd get a Frontier for DD and move DS to the Marathon (assuming it is not close to expiring).