In "seriously" deep mud, a so called mud-terrain won't do any better...or anything for matter.
Yes, the voids are larger than all-terrains, but still not large enough for the a lot of mud. So don't think a mud-terrain will be the end all mud tire. I find myself (AT's) getting stuck just as much as MT's in the eastern style trails (rock with mud inbetween), because the MT's still aren't aggressive enough to self clean.
I'd like to say I'm being biased because I have AT's, but it's what I've experienced in the 3-4 years of wheeling with AT's. So if you are doing "seriously deep mud" skip anything that has the word "terrain" in it's name.
_nicko_