What is the Best Cat food for stomach ?

My cat seems to have a somewhat sensitive stomach. I have changed his food (little by little) and he seems o.k. for awhile then,always in the middle of the night he throws up a little. No food except a couple of chunks, is in it but its a smooth matter. I don't know what to put him on but don't want to spend a fortune if I can help it because my other cat is on a allergy food.. Or is there something I can put in his food for this ? He is on Purina One right now.Thanks