Dear SLoViper,
It depends on the fish and also what they are used to. For instance, it is recommended that goldfish eat sinking pellets to prevent them gulping down--goldies are piggies--flakes on the the surface and swallowing air.... Some goldies have trouble with pellets at first--not looking for them sinking--because they were raised with flakes. But because they are piggies, they adjust rapidly.
My tropicals are another matter--parrot, tiger barbs, tetras, platyies. I inherited them from a friend who was moving. They were used to flakes. I valiantly tried to switch them to pellets. They never got hungry enough to eat the pellets before they became decomposing litter on the bottom of the tank. So I relented and are back to flakes.
Check out the labels, ask friends and see what works for you.
Chris