I was looking at MediaWiki, and came across this note about rsync being fairly ineffective (because of the compression).

http://en.wikipedia.org/wiki/Wikipedia:Database_download#Rsync

So, just in case you weren't aware of it, there is (at least) one compression program that doesn't have this drawback. There is a gzip patch implementing a special --rsync (or, in older versions of the patch, --rsyncable) mode in which it restarts its dictionaries according to the behavior of a running checksum. This does reduce the compression ratio slightly (generally somewhere near 2%), but it also results in unchanged sections being unchanged after compression as well, which means rsync can skip them effectively. The resulting files are fully compatible with existing decompressors. Debian's gzip includes this patch (as do many other distros, I think) and it is planned for inclusion upstream soon.

I don't know how badly gzip fares vs bzip2 on your content, but if it's not a landslide having rsync work well might be worthwhile...