Yeah very different platforms (totally apples and oranges).
I have a ton of experience with the Tarpon 120 and none with a hybrid style kayak, such as the Ultimate FX.
The Tarpon is a great platform for multiple applications, but not a standing kayak. Not so great for heavy storage, either. Great for longer distance stuff.
The FX will carry pretty big loads, allow for stability and standing, paddle well over decent distances, and offer comfort with the frame seat.
The big difference I see here is safety.
The FX will swamp and take on water if you get in trouble, so make your decisions based on that. The Tarpon will give you more safety.
You don't want to be in the ocean, by yourself, and get in trouble. You don't want to be in freshwater, either, and get in trouble. But, you have to weigh out your "style" with your platform decision.
For example, I fish freshwater exclusively, and I usually stay close to the banks. I wouldn't mind swamping an FX kayak because I wouldn't be in any grave danger.