If its a 50GB SAP system I assume that most of that size is an Oracle database?
If thats the case then Oracle must not be active when you make a DR copy of it.
Oracle has lots of files open and keeps making changes. If you are making copies of it over a several hour window then your copy is likely to be junk (especially given SAP likes updating Oracle).
I think IXOS is an archiving solution isnt it? If the IXOS archive is on a unix filesystem and is not an open database then you can copy those files whenever you like. The files in an IXOS archive are almost never going to change so using a utitility like rsync is a possibility. Rsync will only copy the stuff that has changed since the last time it ran.
To answer your actual question. Both rdist and rsync will only copy the files that have changed. Rsync is considered to be better in most circles and comes with the ability to only copy the blocks that have changed in files rather than copy whole files (there are cavaets on using that functionality though). Rsync has more functionality in general.
If you ran rdist to an empty directory on your DR machine then it would have copied the entire filesystem. Try running it a second time when there is already a full copy in place on the remote machine, and it should only copy across and changes since the last time it ran. Use rsync, not rdist though