I've heard good things about both but have neither. I have heard more recommendations for the nautilus on thebump though. In summary, I am no help whatsoever
Don't have either of those, but have two Recaro Convertible's (Como) and absolutely adore them. DD is almost 3, just under 30 lbs and still rear facing, but we still have a year and a half till the seats expire, so we'll hang on to them till she either outgrows it (not likely) or it expires. It rear faces to 35 lbs, forward faces to 70-75 lbs with the 5 point harness. It has a tall shell and DD will probably outgrow it height wise long before weight, but I still really love the seat. It's easy to get a tight install, easy for her to get in and out, and is comfy and safe. Unless we decide to buy another seat with a higher rear facing weight limit, I'll buy a Recaro for our next seat as well. FWIW, it fits well in my car in the middle in the back seat, a Jeep Commander SUV and we have to FF the seat in DH's car, which is a 2 door sports car. We *could* RF it in his car, but she rarely is in his car and it's hard to get her in and out.
We were just comparing the Nautilus and the Britax Frontier. Ended up getting the Frontier because it has a higher limit for height and and crotch space with the strap. If you are thinking about the Nautilus, I heard they are coming out with a new version (Something like Argos) in the fall. GL! I have a lot of friends who love the Nautilus, but our son is on the bigger/taller side, so we wanted to make sure there would be enough room and harness as long as possible.
Re: Convertible/Booster Question
IO have a pinterest board where there are some comments on the Nautilus, I believe. I may have the Recaro on there, too, but I don't recall.
Big E (6) & Little E (2.5)