I built the Shearwater Atlantic, designed by Nick's brother Eric. It's a high-volume kayak and well-suited for someone your size. Initially I built the boat for long expeditions. I am only 5'7" and 155 pounds, but the boat offered a lot of room and weight capacity for gear. It would be ideal for a big paddler. The deck is high enough to give plenty of leg room. My 250-pound, 6'6" friend found it comfortable. As for rough seas, my daily sea forecast is 4 to 6 feet. I have used it for long open-ocean crossings and can attest to its ability to handle big seas. Certainly this is a tried-and-true boat for open ocean. Honestly I think the Guillemot L would be similar to the Atlantic but with a lower rear deck. Just looking at pics- never paddled the Guillemot.

As for stability, I often stand on my rear deck to scout ahead for boat traffic when crossing between islands. I can do this fairly easily, and also re-entry after a wet-exit requires no fancy float setup. I just grab the coaming and throw a leg onto the rear deck, and then climb aboard and back into the cockpit. It has a lot of secondary stability.

Below is a pic of the boat climbing a wave of near 8 feet. Seas that day were 6-8 according to NWS, and the area I was in is where they stack up along a ledge near the shore. You can see how the bow rides up the wave nicely. This is a 19' kayak and I snapped the picture as the stern was at the base of the wave.

Anyway, sorry I can't compare the two boats mentioned, but thought I would give info on the Atlantic since it fits your needs, and it is very similar to the Guillemot L.