These two diagrams contain a lot of the same information and tools like Rational (IBM) Rose can convert one diagram to the other. Most find that collaboration better communicates the relationships between objects while sequence better communicates lower level details.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi
Sequence diagrams are good for showing sequential logic, I like to characterize them as visual source code although they can be used for higher level logic too.
Collaboration diagrams, in UML 2 they're called communication diagrams (sigh), are good for showing a bird's eye view of collaborating objects.
They're both good diagrams. Depending on what you're trying to understand, and the people you're working with, one will work better than the other for you. Sequence diagrams seem to be more popular, but they're both useful.