Ok I have done a tone of research on this topic. I have a 2 and 1/2 year old and another on the way due in 3 weeks. We have always has a britax for our child. She had a companion when she was an infant and then a Marathon now. I love Britax! However with that said I have looked at two five point harness seats... The britax Frontier and the Graco Nautilus. Here is what I have found out.
Frontier is great but you can only use latch until 40 pounds and in some vechiles it has issues with getting a tight secure fit using the seat belt. For example in the Honda Oydessy it only gets a secure fit in the 3rd row. Britax is aware of this issue. Hard to install. The arm rests do go up and it is great for a child with a larger frame. There are many adjustments for the bottom belt strap to keep them in the 5 point harness longer. And can be used with the 5 point harness until 85 pounds (It is 100.00 more expensive than the Graco).
Graco- Better for a smaller framed child. Easy to install correctly with a tight fit. Arm rests do not move. However goes into a five point harness booster, reg. booster and the back does come off for a backless booster later on. The con on this one is the bottom strap does not have as many positions as the frontier so even though you can use the 5 point harness up to 65 pounds. Most children out grow the bottom strap before then and are using the seat belt.
I have tried both boosters with my daughter and installed both in my car (Honda Accord) For the price and fit I was able to achieve we are going with the Nautilus over the frontier. I found the Nautilus on sale at Walmart.com for $125.00. The cheapest I found the Frontier for was 275.00 and when you put the two side by side I don't see a 150.00 difference. And I was not able to get the secure fit we needed from the Frontier. My daughter also seemed to like sitting in the Nautilus better and it takes up less space in the car. I hope this helps.