I made an 830 all-plate ret paladin and it was trash. I ended up leveling him up to 109 and made him a BG alt since he still performs well there.
I main a 110 UH DK and I can tell you that unlike many other 101 twinks, it will take several seconds to build up your damage. I'm sure UH has great sustained and aoe... if the target(s) lives that long. The problem with 101s is that stuff dies so quickly you will not see the full potential of UH. This, of course, is simply my opinion. I also play a 101 frost DK which absolutely shreds.
My advice, if you have to choose one of the two, is UH. You will probably have to get your ilvl up a bit higher than other classes but with some leech you can pull tons with your plague and burst down massive amounts of mobs if you stack them.