My advice would be to go with the NC. I of course am prejudiced, because I own an NC Expedition. I have paddled a lot of other boats and own other kayaks, but if I could have only one, it would be an NC.
I can answer a couple of your concerns. The NC design is extremely stable. It is the most stable kayak I have ever paddled. All NC kayaks have the same seat, which is fiberglass and very, very comfortable. When I first got mine, I thought I would need a soft pad to be able to spend a lot of time in the seat. I couldn't have been more wrong. I used the pad my wife made for me a couple of times and that was it. It has been my experience that any kayak seat takes a little time to get used to, but again, of all the kayak seats I have sat in, the NC would be my choice--no hesitation whatsoever.
You question whether composite's are superior to plastics. That isn't so easy to answer, because there are pluses and minuses to most building materials. If you are rough on your boats and are prone toward abuse, thermal plastic and some polyethylene boats might be your choice. However, composites are very hard to beat for stiffness, and just plain feeling right. If custom coloring is your thing, composites are certainly the way to go.
NC can offer a choice of other fabrics, but they will carefully explain to you the advantages of fiberglass. I chose fiberglass and if I had to do it again, it would still be fiberglass. The layup that NC uses in their Lt boats results in a very light, but rigid boat. My Expedition weighs about 50 lbs and is the easist boat that I have to carry on my shoulder.
NC describes their boats as being very solid trackers; they are. That doesn't necessarily mean that they are hard to maneuver. If you have to have a boat that turns on a dime, NC is not that boat, but be assured, nearly any boat can be nicely turned with the right techniques. However, not every boat can make paddling an absolute pleasure when it comes to racking up the miles. That is what an NC will do with speed and comfort and extreme good looks.
If you are looking for unequaled quality for your buck in a new boat, you'll be hard pressed to beat what NC has to offer. At the present time, NC is having a sale. I noticed their boat of the week--a seventeen footer is going for about the price of a good poly boat. That should be a no-brainer.
Anyway, if you have any further questions at all, give NC a call and talk to Doug Searles, or Greg and I'm sure they will help you out. I will also be glad to answer anything I am able to.