I think both are great choices, but I also don't think either company got it quite right:
I like the Kel-Tek set up which allows you to choose between two types of ammo, but I wish they had the 'option' to automatically switch magazine tubes when empty for times when you are only using one type of ammo.
As for safeties - cross bolt for the KSG vs. non-ambidextrous AR style for the UTS-15. I would have prefered both models to be designed with an ambidextrous AR style safety like the RFB...
Given the options I chose the KSG over the UTS-15 because bullpups really need to be ambidextrous. The KSG is fully ambidextrous, while the UTS-15 is not ambidextrous at all, although you could at least fire one round from the opposite side, but you would then need to swith sides before cycling the action again.
I do have a question on loading the UTS-15... you say loading the UTS-15 is faster? I would understand it being easier with the tubes on top compared to the bottom, but doesn't it take longer when you factor in the need to open and close the top cover every time you load? I could understand it being faster overall if you run empty and then do a full reload all at once, but I would expect it to be a lot slower (with all the unshouldering, opening and closing) if you wanted to toss in an extra round or two every so often. Some more detail on the loading would be appreciated.