what do you want the dog for? If you plan to show, either would be fine.
If you want a working dog, I would go for UKC and find a good breeder who actually works their dogs in what you are interested in. My internet is too slow to research this morning.
In AKC, the belgian is separated into 3 different breeds - the rest of the world, it's one breed, 3 varieties. I'm not sure how the UKC, but I think they register as 1 breed.