Below is the Captain's Chair that I find the most comfortable. And you want a comfortable seat to row from, and to sit in as you float. I've had two back surgeries, so the seat I sit in is very important. If my back were to go out during a multi-day trip, it would be major trouble. Not only due to the need to get the boat and my family through the canyon to the "take-out"...but all the work involved in setting up camp, unloading gear at camp, reloading in the morning for the next day of floating...a bad back would really be a problem. So, I'm a believer in having a seat that works. Below is a pic of the seat I like, go hear to read more: NRS High-Back Seat
I have found the high back Captain's Chair to be the best in providing comfort and lumbar support. I don't think anything is lost as far as performance. Below is the low-back, which some people feel provides better performance in that you can lean back more as you row.
I've used both and highly recommend the high-back. I'm sure there are times during hard-core rowing that the low-back has it's uses. That said, I've rowed some long days and down some pretty technical runs and have never had the higher-back get in the way.
Above is a pic of a different type of seat and frame. Many rafters like this type of high-back seat that has a mesh fabric (that can be replaced). I don't find these seats as comfortable, but many rafters love them. If you're in the Denver area go to AAA Inflatables or Down River Equipment to sit in one. As I said earlier, Mark at AAA does amazing work and is a great resource! This pic also shows a different type of frame than the NRS one I have. It has nice diamond plate decking and pretty much rocks! Below is a better pic of this type of seat. It's made from 1 1/4" pipe.
Regardless of what type of Captain's Chair you end up with, you'll want to make sure it is both comfortable, and allows you to row effectively. I've spent a lot of time in both types of Captain's Chairs, and find the high back NRS to be the by far the most comfortable for long days in the seat.