DEK1 was designed to function as a hunting knife- not specifically- but I kept that use in mind during conception. If you were to use it for that and fishing I'd highly recommend mounting a lanyard as the rear of the handle doesn't have a pronounced beak to grab on to if it's going to slip out of your hands. Other than that, the unobtrusive handle allows for a lot of different holds and to move from one to the other easily. Also, the high s grind and dropped point provide a nice svelte edge geometry and good point power and control, especially if you like to put the tip of your index finger up near the point.
FK1 was an amalgam of Nathan and my ideas on a compact, super tough minimally sized but useful for most stuff kind of knife. If you are parting out an animal, you might find the scraper at the butt of the handle to be a real problem. If you find yourself having to smash out a window in a sinking car or cut yourself out of the fuselage of some flying thing, or treat your knives like common tools, FK1 will be with you to the end. The scraper feature is super handy for popping tops, so if you like drinking beer you'll have an opportunity to use the knife without removing it from the sheath, which is especially handy if you've had a few.
They may be the same size, but in my view they are very different knives and if you have both it might be worth considering holding on to them until you've had enough time using them to figure out which one works best for you.