You can have both: 
The 26 rear wheel of the Gekko 26 which is great for speed and also partly for stability.

And you get still a foldable trike that is relatively easy to transport. 

Yes, it is a bit bigger (and heavier) than the HP Gekko fx 20, but since transporting a foldable trike requires a larger car anyways, this isn’t much of an issue.

Just make sure that you are ok with having a trike without a suspension. For me, I believe if you find the Gekko fx 26 more attractive compared to the Gekko fx 20 because of the higher top speed and smoother run, then these are exactly some good reasons to look for a Scorpion fx 26. Given, that trike is a bit more expensive.