We have the britax frontier, which has a higher five point harness limit of 85lbs. Then uses the seatbelt to 120lbs. Because five point is safest that's the main reason I chose it, but also because of britax's safety ratings and personal experience with their brand in a fatality accident.
I've heard so much about the nautilus that I went and looked at it for our second DD, but it seemed so cheap, fabric wise and shell when compared to the more substantial and cushiony britax seats. We will spend the extra and get another frontier for her next year. Safety is worth a lot to me, even the extra $50+! plus that makes up for how much longer they can use the harness. There's always good deal on albeebaby.com or new in box on eBay.