I have the Nautilus and love it. Just like some other post said about the cubby my daughter loves them. I have a scion XB and I do not have any problem putting in or taking out. It is really easy to take apart to clean. The 5 point is up to 60lbs and I love that. I have told many people about it. I can not think of anything bad about this seat.
I would go with the Britax Frontier85. You will be able to harness much longer than with the Nautilus. The top harness slots are at 20" with the Frontier85, as opposed to 17.5" with the Nautilus. That 2.5" makes a huge difference. My girlfriend bought a Nautilus thinking that this was all she would ever need, but her daughter is 6, and has already outgrown the harness. She is still fairly light though and would like to keep her harnessed as long as possible. So she sold her Nautilus and just bought a Frontier85. My son is 6 and still has room to grow in the Frontier85, and he says it's really comfortable. But I did have my doubts about it fitting well in my car since I can no longer use latch for installation (latch only goes up to 48lbs in my Honda Van..and my son weighs 50) and I have to use the seat belt to instal. I was able to get a great tight install in about 10min.
I would stop by a BuyBuyBaby, and check them out side by side with your kids and your car. They have both in stock. They were soooooo nice there, and it was great to be able to see my kids in the seats and see what worked. And if for some reason after you buy it and it doesn't install well, they will take it back no problem :)
Hi Beth - Hands down the Britax. Extremely safe (higher safety standards than set by the US government) and easy for a parent to install and buckle. We just finished using our Britax Marathon and went to the Britax booster. The best investment we will make in our little one! If you are interested in purchasing a very gently used Britax Marathon, give me a jingle (but neither one is pink ...).
Thanks so much for all of your advice and comments! Since the Graco actually got some negative feedback and the Britax got none, we went with the Britax. I figured that since this is literally the last carseat my daughter will ever have, it was worth the money and in the end, i found it with a 15% discount, free shipping and no tax online, so it wasn’t all that much more than the Graco anyhow… AND it’s pink! She loves it and tells everyone about the cupholders 
Also, i read the manual throroughly and didn’t find it terribly hard to install (it’s the 85, so they’ve made some improvements, I guess). It was just a bit different as I’ve never used the anchor thingy before so there was a bit of jumping around in the car to make it all work right. But we’re very happy with it, so thanks for all of your feedback! Super helpful!!