Crumbles are just pellets that have been crumbled up into smaller pieces. I used to use crumbles, but switched over to pellets as I can get them $4 cheaper per 50 lb. bag. Also many say that pellets result in less waste of feed, as any pellets that may fall onto the ground can be more easily found and eaten by the chickens.
I like the pellets because they don't waste as much. However, I have read threads where peoples birds have not eaten well when they first switch so if you do change keep a close eye on your birds for awhile too make sure everybody is eating. It would probably be a good idea to mix the pellets with the crumbles at first so they get the idea that it is food.
Pellets are best for big chickies, but definitely crumbles for bantams. I've gotten pellets and some of them were over an inch long and had to break them up so the bantams could eat them. Most of the time I find that crumbles and pellets are the same price.