Re: UK4AA, Pelican Stealthlite, or SL4AA ProPolyme
Unfortunately I don't have personal experience with all 3 lights. I do own a UK4AA, and for what its worth, it's been my favorite incandescent task light since.
PlayboyJoeSchmoe has already explained much in detail. The switch is easy to use, being in the appropriate place, and the light itself is actually quite comfortable. The stacked battery design and small bezel make the light very pocket-friendly. The output is very decent, especially for $16.
I originally bought mine after a member recommended it to me to replace my Surge as a robust, high intensity light for finding cairns/trail blazes in harsh weather. I've since given them to my father and uncle as christmas presents, and sent my cousin and close friend over to Iraq with them. Although not flashaholics, they were impressed and have been quite happy with them.
I don't know if there are any advantages to running an LED module (like the SL PP 4AA 7LED) with Ni-Mh, but if you're going incandescent, I can give the UK 4AA a healthy thumbs-up.
Oh, I don't know about the other 2, but the UK is supposed to be water resistant to 10', and has the nifty gas vent in the back.
good luck!
-Rail