I enjoy both -- there's not a single inch of the female body I don't adore or have reservations about kissing. Cleanliness is a must, but that really ought to go without saying.
If you're going to bring up an unclean ass in this thread, then it's only fair that you bring up other unclean body parts, right? Anyone for some bloody, urine-sodden pussy? How about kissing a mouth full of open sores and yellow, rotting teeth? No? Well what about about sucking on those fingers that have been handling raw meat or just scratching the dog?
With regards to disease, while some sexual activities are riskier than others, pretty much everything has some risk attached to it. It's up to each individual to decide what constitutes an acceptable risk. Licking a properly cleaned ass is nowhere near as high risk as has been suggested by some in this thread, assuming your partner doesn't have something like HIV or hepatitis.