I've packed with both horses and mules, and it's true that they entail a lot of overhead. But when you want to haul a lot of people/gear into the back-country and a lot of game out of the back-country they're nice to have. I think both horses and mules have their ups/downs, but a good mule is worth its weight in gold while NOTHING is worse than a bad mule.
A bad mule who likes to buck/kick/bite/jump fences isn't worth having even if he can pack the kitchen sink, in fact they can cause a lot of trouble and expense. They'll even kill new foals and calves if you happen to have any around. If you can get your hands on a good calm mule with packing experience that's also broke to ride, it's worth the $5-$10k you'll spend on it. Another nice thing about mules is their economy and longevity. I'm sure there's no exact figure, but mules can subsist on much less feed and lower quality feed - sometimes 30% less than a horses require. Also, if well taken care of, you can get 35 years of use out of mules. We've got one Molly Mule that is 38, we stopped packing her last year but we can put ANY rider on her and we never have to worry. If you can, buy a mule from an experience packer so it's had lots of miles with a pack saddle on. You can even buy mules from the folks that run tours at the Grand Canyon - now those are dead broke animals.
Horses are a case by case situation like mules, but in general do loose their cool quicker than mules. They also tend to get wrapped around trees more often too - both animals will step off the trail to grab a bite while packing, but mules are smart enough to get back on the trail before letting a tree get between them and the animal they're tied to. But if you know where and how to look, you can find a nice 'dead head' type of horse for less money than a mule that will quietly pack anything you put on it and never cause trouble in the pack string. I've used lots of horses like this, and they usually aren't much for riding but they make great packers.
