Amazon SimpleDB or Amazon RDS?
Well it's really a matter of you deciding on the architecture you want to go with.

SimpleDB is a non-relational "no-sql" data storage system built around the ideas of collections of items accessible via key/value pairing.

Where as RDS is really just a custom version of MySQL which a fully relational SQL database.

both systems will work for tracking profiles but the implementation patterns are very different. Neither is right or wrong, so it's your call on what you want to do.