The pretty bird ones are more textured and crunchy looking. The zupreem look more smooth and the colors are brighter while pretty bird is a duller color. Also the zupreem have a much more fruity smell to them. Basically pretty bird is somewhere in the middle between the colored/flavored and uncolored sorts of pellets.
May I suggest putting Scotty on medium Roudybush and then serving Scooter his leftovers? I'm not sure if Scooter is big enough to eat the medium pellets himself (Kili does and loves them). But if he isn't, I guarantee you Scotty will leave behind a load of smaller pieces in his wake and they would be just the size for Scooter. No need to buy 2 different sizes that way!
