bup backups are 2.7 times smaller than rsnapshot
Lately, he's been working on an rsnapshot-to-bup importer tool. According to his most recent message, after importing the incremental backups from two servers, the bup repo was 4.6GB vs. the original rsnapshot disk usage of 12.6GB. That's 2.7 times smaller.
YMMV. Results may not be typical. Always consult a physician.
As of last night, you can also restore your bup backups. Yeah, I know. It's that awesome.