Re: Nautilus / ProSport / Frontier ???
We have the Frontier and LOVE it. DD is 4 and 30 pounds (she's a peanut) and about 39" long. She has LOTS of room left in it.
The only feature I don't care for is that you need to take the seat out to adjust the harness. But this really only needs to be done 1-3 times a year so it's not so bad. We got it in March and I just raised it 2 weeks ago!
I left the top tether latched and just undid the latch system and slid the seat forward, shoved my hand up the back and adjusted it like that. That way I didn't have to undo the whole seat.
She's never fallen asleep in it (she's not a sleeper in the car) so I can't tell you about head slump.