Sort of "good" advice. ;-)
These answers are never simple, are they? One big advantage of the Marathon is that the shell is larger. That means many toddlers will be able to use it rear facing longer. My daughter has outgrown her Roundabout rear facing, but will probably have several inches of space rear facing in the Marathon. Rear facing is significantly safer than front facing, and for me, that advantage outweighs the price difference. See
www.thematlocks.com/jenni...ndedRF.htm for some *excellent* info about why extended rear facing is a wise choice.
Having said that, I really recommend you wait and try both seats when the Marathon hits shelves this month. You may find that you really like one better than the other. The Marathon will have Britax's "HUGS" system, for example It will also have a different harness adjuster than the Roundabout. They will have different bases, and one might fit in your vehicle better than the other. And, there are a wide variety of other factors which could sway you one way or the other.
As for the expiration issue, yes, Britax recommends you replace their seats after 5 years (other companies vary, but all recommend a 5-8 year life span). Your seat will probably "expire" before your child totally outgrows it (Britax designed the Marathon to fit most children until they are at *least* 6 years old). A Roundabout will probably be outgrown before it expires. Either way, you'll need to purchase a new seat in the next 3-5 years. Would you rather spend $50 less on a Roundabout now, but replace it 1-2 years sooner or spend $50 more on the Marathon and use it longer, but replace it before it's totally outgrown? *I* think the two cancel each other out. Your situation may be such that you feel otherwise.
Finally, the technology involved in child passenger safety is advancing by leaps and bounds. When you compare the seats and features currently available with those available just 5 years ago, the changes are staggering! Even the past 18 months has brought many new things to fruition. If the same is true 5 years from now, there will probably be many new products with even *more* advanced safety features available, one of which will probably meet your needs at that time.
I know it's not cut and dried, but I hope I brought up some issues that will help you make the right decision for *your* family!