I can only say that Britax is really solid and always gets great reviews. My son and daughter-in-law did the research and as a result there are six Britax car seats in our family for two grand-kids! They seem to be solid and safe. There are pads for comfort and easily adjustable for changeable weather and heavier/lighter clothing.
I don't know about the Nautilus, but my daughter says that this is a comfortable seat. She is 4 and we just switched to the shoulder harness mode. I do have to say that I don't like the way this seat installs with the latch system as it is hard to get tight UNTIL I found out you had to thread the tail of the strap up through the seat and pull forward from the seat instead of on the side where the tightening strap is located. That made all the difference, then it was simple to get installed. It has great padding and I love the straps that stay straight and are comfortable.
I have both... We put the Frontier 85 in my suv and the Nautilus in my wife's suv. We have a summer home that is 350 miles from our home and we always take my car. He is clearly more comfortable in the Frontier. The Nautilus isn't bad, it just doesn't seem as roomy for my son. The straps on the Nautilus are extremely short which make it difficult to use at his size, which forces us to use the seat belt. With the Frontier we will be able to use the harness for quite awhile longer....clearly safer. He is 5 and weighs about 40 lbs. Hope this helps.
My 4 yr old loves this seat. The padding is comfy and he often lays his head on the supports to relax. I wouldn't get this seat for younger than 3. It sits straight up and there is no way no recline it. My son doesn't sleep in the car but he did in his Boulevard Britax seat when he was younger.
I don't own the Nautilus, but the Frontier 85 is outstanding. The cushion/lining is soft and seems well padded for a car seat. The harness has the HUGS system, making it more comfortable by design. The harness connector has a belly pad, and also has 3 different positions for a comfortable fit between the legs. The head rest is also nicely padded, and is far enough from the sides of the head to allow visibility, but close enough for a child to comfortably rest their head if they get sleepy. The seat is easy for a child to get in and out of, and the straps are super easy to adjust - both to tighten/loosen and to adjust up or down for height. As I'm typing this, my 5 year old said to tell you that his seat is very comfy and he likes it a lot. :-) I am very pleased with all aspects of this seat, including installation, and I plan to buy one for my daughter later this year.
I have the britax Frontier 85 Combination Booster Car Seat, and I can say it was a great purchase. It's very comfortable for my 2 year old son, it satisfies all my safety needs, it has the advantage of the cup holder (witch are great, less spills in the car) and the option in colors where good too....