Like I wrote in the first post about the 18XS, it was due to logistic issues not possible to test the 18x.
In theory it is mentioned by Kanoshop.nl that the XS has a better stability. Besides this no big difference in performance between the two kayaks was expected. The only visual difference is that the cross-section of the 18XS is a bit squared, giving more volume and stability. Other parameters are said to be equal.
As quite a lot of paddlers are interested in the 18X (because it is said to be faster), I was very eager to know more about the difference with the XS.
A seakayak-test-day, organized by 3 suppliers, was a present as I could easily switch between the two kayaks. Even better was the weather cooperating: It was rainy weather and a 5-6 bft wind was blowing over the Grevelingen where the event was organized.
Although it was not possible to paddle at sea, the weather conditions at the lee-shore were heavy enough to get an impression of the performance.
I think that the difference in stability is only marginal under the current weather-conditions: the 18xs being only slightly more stable than the 18X.
I paddled as much different courses relative to the wind- wave-directions and to my view all other characteristics were very much the same for both kayaks.
Besides this I noticed a significant difference which is a bit related to my constitution. But is is worth while for potential buyers to be alert on this issue:
It is about the feet-rest and steering pedals. The cross-section of the 18X is a bit narrower than the XS and the footrest and -pedals are mounted a few cm higher than in the 18XS. For my feet it was difficult to give full left or full right with this rudder, necessary to keep course in a following see for example. I had to lift my heels from the bottom and stretch my feet to the maximum causing cramps in my calves.
It is obvious that someone with bigger feet does not have problems with this difference. So be alert if you trie one of those kayaks.
An explanation could be that the same footrest is mounted in 18X as well in the 18XS. In that case it is easily solvable to my opinion if Epic is willing to mount another footrest, if necessary, in your case.
May be the small difference between both kayaks makes the choice between them more difficult.
Nowadays a lot of paddlers want to have a fast kayak. The 18X as well as the 18XS have both a high speed potential. BUT it is the question if YOU can use this. Kayaks are said to be faster when the waterline is longer. This is certainly true, but what most paddlers forget is that a longer kayak has a greater surface causing more friction in the water than a shorter kayak. So, if you want to go fast with a long, fast kayak it is certainly necessary to have the power to speed up the kayak to its limits. Meaning that you have to train to get the power and the technique. Something that I do not see amongst most seapaddlers.
Not that an Epic 18X or 18XS are bad kayaks. On the contrary: they perform very well 
and you can keep up easily with more conventional seakayaks. But real fast with an average seapaddler aboard? I don't think so!