What is a LSN (log sequence number)? How does xtrabackup Incremental Backup work?
LSN a.k.a. Log Sequence Number is a 8-bytes integer that acts as a version to REDO log entries(ib_logfile*). Every change to data makes an entry in the REDO log and gets a new LSN. The LSN is incremented with every change.

Every InnoDB index page (where user data are stored) has the LSN field in the header. It's an LSN that last modified the page.

So. If we know what the last LSN (let's call it lsn_full) was when we took a full backup, we know if a particular page changed since then. Because if the page's LSN (let's call it lsn_current) higher than lsn_full, the page has changed since the last backup.

Xtrabackup uses this idea to implement incremental backups. As a matter of fact you can implement both incremental and differential backups with Xtrabackup. Check out options --incremental-lsn and --incremental-basedir.

Whether use incremental backups or not depends on how confident you're about the whole process. It's easy to mess up if you don't fully understand what's going on during the backup and restoration process. (At TwinDB I have to deal with lots of cases where incremental backups were not usable on whatever reason).

I highly recommend verifying backups. The most reliable way to ensure a backup copy is usable is to restore the database from it. So I would suggest algorithm:


grab the latest copy.
restore a start MySQL instance on it.
Check some table if it contains recent data. For example, if you store transactions in the database make sure there are some not older than a day or something.