The biggest difference is that the crook knife is more or less a one-handed draw knife. You hold it with your hand upside down, thumb straight out, and draw it toward you. Works OK for carving out the interior of bowls but it's also useful for finishing carving/planing on projects like oars or things of that nature.
Spoon knives are pretty much best suited for carving out bowls and spoons. They are very handy at it.
I'm not much of a carver so that's my experience with the two kinds of knives. For my uses I prefer a crook knife as it's more comfortable for me to use. If you do a lot of carving both have their uses.