I have the new Trident 13, and a guy I go fishing with has the Trident 15. Both are great kayaks with awesome features for both inshore and a few BTB trips. The 13 was my pick because I was able to try out his 15 first and it was just a little too long for my liking. It was a smooth paddle, but not easy to turn and maneuver in the marshes and flats. The 13 is just as smooth to paddle in my opinion, but also able to maneuver in the flats. The other only main difference in the 15 from the 13 is the back storage hatch. I think the front and the middle mod pod allows plenty of storage, so the extra hatch in the back would go unused 95% of the time.
Also think about handling and transporting the kayak, believe it or not, but the extra 2 feet of kayak makes a big difference when needing to transport it alone. I have a bed extender and am able to load my kayak alone in the bed of my truck when it is fully loaded with all of my fishing gear. My friend loads his on a truck rack, and he has perfected loading and unloading it alone, but I can tell it took some time to be able to do that, and he takes everything out of it before loading.
I do not think you could go wrong with either the 13 or 15, both are great kayaks that would last a lifetime if taken care of.
I would also recommend the ice box that fits the tridents mold, especially if your going BTB, it will stay secure and not fall out.