I think the reason for choosing UKC over AKC would be as varied as breeders are. They aren't one of the 'alternate' registries that have sprung up lately. UKC was founded in 1898 and have primarily been focused on performance events. They now do conformation as well.
The one plus about them is that they've done nothing major to tarnish their reputation, at least as far as I can tell. Can't say that about AKC.
Here's the link to UKC..
United Kennel Club: Home