If you are transporting SAS data library files, there is an XPORT format required for certain OS platform conversions/transfers. There are coding examples on the SAS support http://support.sas.com/ website. Suggest whatever approach you choose to exercise it liberally with some testing data scenarios.
I'd tend to use gzip. It's available, reliable, and free.
How big is big? Will the Sun file system handle files that are as large at those on AIX. That is often operating system version dependent ("Sun 64" is not sufficient. I think that you need Solaris 10 to get to the really big files.).
Is the data already in a SAS dataset? If so, make sure the Sun 64's SAS will read datasets created by the AIX ("cross platform compliant"). You may want to consider converting to a SAS transport file first.
Disk space. Is there enough disk space on both to do the work. If disk space is a concern, you may want to invest in an FTP server that will compress and uncompress on the fly.