Im currently using scp, but it seems to become slower and slower every day. Im up to 30 servers, and I need to shuffle around 3-5k files per day, with the average size of 200-400MB.
I used rsync before also, with even more pitiful results.
Each server should be able to transfer files to any other server in the pool (Im adding about 7 servers a month now), so key management is pretty crucial also.