I can't help you decide between the M3LT-S and the UB3T, but let me tell you what I did:
I had a UB3T on backorder, payed in advance, but put the order on hold when SF anounced the UBR at ShotShow 2011.
The UBR is planned to be released by the end (October-December) of this year. Until then I will wait because I would like to be able to know more about the UBR once it comes out. If the UBR is released on time, it will be a tough choice. I am especially interested in the runtimes. And if the UBR does indeed accepts (next to rechargeables) two primaries (not three which I would prefer).
I would like to be able to compare those lights before I decide. It could very well mean that I go for the UB3T, but only after a comparison.
And the release of the UBR get's pushed back like they did with the UB3T, then I will likely get a UB3T after all.
so, wait if you can. The UB3T is just out and will be available for the next couple of years. No need to rush, unless there comes another financial crisis.....
(being a flashaholic it's not so easy to resist a long wait haha)