Which version of xtrabackup to use with MySQL 5.5?
Percona XtraBackup 2.3 should be backward compatible, so you should be able to use PXB 2.3 with MySQL/Percona Server 5.5. There are some edge cases where you will need to use PXB 2.2 or even 2.1 but it is rarely needed if PXB 2.3 works.

PXB 2.4 is recommended to be used on MySQL/Percona Server 5.7.

In any case, you should always test your backups so backup using PXB 2.3 and restore it on another instance to see if everything works.