An anal eater is ... well ... some person (or biological entity) which uses its anus to eat. Planaria are anal eaters, for in planaria, the mouth is also the anus. I don't think most human beings have mastered this trick yet, but if emergency room records are any clue, there are plenty of resourceful individuals striving to make headway in this difficult field. (So, Mr. Smith...tell me again how you managed to get a kumquat stuck in there?) (It was one in a million, Doc! One in a million!)
An anus eater is, quite clearly, some person (or biological entity) which uses its mouth (or anus, or other gastroenterological part) with which to eat anuses. Or even a single anus. It should be obvious to everyone that anyone who eats hot dogs or sausages or cat food (or probably hosts of other delicious foods rich in bi-products) is an anus eater. Also, in all likelihood, a lip, feather, and beak eater. (Awww...you can really taste the anus...)
It would be interesting to see how much overlap there is between these two groups. I'm guessing probably quite a bit, as it seems to me that any anal eater worth his or her salt is going to be drawn to the prospect of sausages or hot dogs sooner, rather than later, in their anal-ingestive career.