at the moment mysqldump (22 minutes exec time, 5 GB Dump), when to use xtrabackup?
Can't you test this on a test machine? I don't mean to be rude, but only you can really tell if XtraBackup is better  for you. What I do know is that  Percona is a quality outfit and that their offerings are always worth a look. 

I'm guessing about your scenario, but any backup will depend more on your disk config than on a particular programme (RAID config &amp;c.). There are four things to be considered in any IT setup - hardware, networking, OS and software (and maybe 5 - people :-) ).

Have you also looked at MyLVMBackup? Zmanda? If you're considering XtraBackup, you must be on Linux - these also work on that platform.