Is it better to use an uniqueidentifier(GUID) or a bigint for an identity column?

That depends on what you’re doing:

• If speed is the primary concern then a plain old int is probably big enough.
• If you really will have more than 2 billion (with a B 😉 ) records, then use bigint or a sequential guid.
• If you need to be able to easily synchronize with records created remotely, then Guid is really great.

Update
Some additional (less-obvious) notes on Guids:

• They can be hard on indexes, and that cuts to the core of database performance
• You can use sequential guids to get back some of the indexing performance, but give up some of the randomness used in point two.
• Guids can be hard to debug by hand (where id='xxx-xxx-xxxxx'), but you get some of that back via sequential guids as well (where id='xxx-xxx' + '123').
• For the same reason, Guids can make ID-based security attacks more difficult- but not impossible. (You can’t just type 'http://example.com?userid=xxxx' and expect to get a result for someone else’s account).