With the spelling Anisa I would pronounce ah-nee-suh. No association with anuses, and a very lovely sounding name. I would think that Anissa was supposed to be pronounced ah-nih-suh. Slight association with anuses, but only if someone suggests it or I have had a few drinks and am giggly. Still a nice name, but I don't like it as much.
Good luck making your decision!