Please don't put your cat on Science Diet! You are correct about there being an ulterior motive. Vets sell Science Diet because they are paid to, not because it is a quality food. Vets are not necessarily schooled in pet nutrition, which is obvious when your vet is pushing cat junk food. Blue and Merrick are both quality foods while Science Diet is full of fillers and by-products. If you search around on this site a little, there are quite a few posts with lists of quality cat foods including Wellness, EVO, Merrick, and Blue Buffalo.
Are you feeding dry or wet? One of my cats has had occasional loose stool (not quite diarrhea but not normal consistency) and canned 100% pumpkin has helped fix the issue (gives the cat more fiber). Have you gotten results for the most recent stool sample yet?