I have a Graco Nautilus for my son and we both LOVE it! I like the look of the Nautilus, but the Frontier is similar in "big kid" style, so I doubt that will help you make the decision.
In my mind, the big differences are the cup holder and the price. My son (4) really does use the cup holder a lot. It's nice and open so he will et his snack or water down there and can get to it easily. I haven't seen a Frontier up close in a while, but I don't think it has this feature. Also, the arms of the Nautilus are solid down to the seat, so he can tuck his book or snack down into the arm where he can reach it rather than it rolling away in the car.
Of course the price is great for an extended harnessing seat!
The only con is that it is REALLY heavy. Obviously not a problem in the car (actually installed more simply and securely than his previous Alpha-Omega) but when we went on vacation recently, I had to buy those go-kid wheels to get it around because there was NO WAY I was going to be able to carry it through the airport. (with the go-kids it was easy to pull and my son could sit down and ride when he was crabby on the way home).
Hope this helps.
T.