S.C.
I debated between these two seats as well. My daughter is 5 and just a bit below average on weight/height. We went to the store and had her sit in both of them back-and-forth, back-and-forth about 20 times. We discovered that the crotch strap on the Nautilus sits way too far back- she was almost sitting on top of it. Even though I liked the price of the Nautilus better, we had to go with the Frontier because of the crotch strap. I really feel this is a design flaw- it needs to come up out of the seat several inches up from where it does. Its position also prevented my daughter from being able to buckle herself. You will see this issue come up A LOT on Amazon reviews.
The Frontier took a few times for her to learn to press firmly enough before she could do it on her own. We have no concerns fitting the Frontier into a Nissan Altima, and she is comfortable sitting in it. She has much more room to grow in the Frontier.