I would go with the WSM(big surprise eh).
I know alot of people who own both, but only use the WSM once they got one.
I actually could have had a UDS for free but dont really dont have any interest in another smoker nowdays.
Like Rick I can get 9-10 hours on a single ring of fuel(I use lump though). Those smokes have all been below in freezing temps, so I am guessing I will be able to go longer once the weather warms up.
WIth the above said I dont think you could go wrong with either, good choice going with a charcoal smoker.