Although these two terms sound similar they are actually very different things and therefore should not be used interchangeably.
An anal eater is someone who expresses some kind of anal-compulsive behavior in their eating habits. What exactly that means is highly subjective and may only be considered as such in the eyes of the person calling them an anal eater. Examples of eating habits that may be seen as 'anal' range from vegetarianism to someone who won't allow any of the different foods on their plate to touch eachother. I once called my mother an anal eater because she refuses to serve common condiments such as cream cheese, salad dressing or even barbeque sauce in their store containers when company is over for dinner. Instead, she empties the contents into some nice porcelain bowl so as not to give a 'tacky' look to the table, only to transport the unused portions back into their original packages which she stashes in a cabinet until the meal is over.

On the other hand, an anus eater is someone who engages in very thorough oral sex practices. Rather than sticking strictly to the genitals of their partner, they go all out (or should I say in?) It is also something I would never call my mother.