None.

The difference between MTS and dedicated server configuration is only in implementation of user's server proceses, which are only memory structures.

On the other hand, backup deals only with "physical" structures (tablespaces and OS files), that is structures that actually stores database information on disks. The backup is independant of Oracle processes and memory structures.

Altough if you are connecting from remote site to perform a backup I'd recommend to use dedicated server connection for that purpose. It should work with shared server connection also, but for backing up the database I'm feeling better if I know I have a connection just for myself....