In theory, the backup made by Plesk should be restorable in a higher version, but this would be risky. So yes, same version ideally.
Another risk with Plesk's own backup is the danger of it not restoring due to some tiny error, and the huge length of time it takes to restore.
Thinking about it, you'd be so much better off using 4psa's TotalBackup. http://www.4psa.com/products-4psatbackup.html
This backs up (tar and zip plus other options) what you need to back up, dumps mysql databases, can do incremental, can do ssh and ftp transfers to remote server etc etc and costs very little. You'd store the backup on the other server. It isn't doing anything massively special, but it takes care of all the details for you so you don't have to worry about them. We use it to backup locally, then copy the backup to Amazon S3 (which is hard and requires doing silly things to achieve).
If server A dies, you restore server A to working, then restore data from your backup ontop of this to get back to where you were.
Alternatively, you can restore your backup on Server B as long as Server B has identical OS and probably Plesk to Server B.
There may be problems with this that I've not thought of. You should consult 4PSA sales about it all in case anything I've said isn't actually correct.
If you do contact them please tell them I sent you so I can get some brownie points www.4psa.com