I'm using "Best" compression for both, so no problems there.
First test for this file shows 3% compression gain with 7zip (306 MB vs 314 MB).
As for rar, 2% compression gain (308 MB vs 314 MB).
File:
http://ia300134.us.archive.org/1/ite..._HQ_part04.avi
For tomorrow's test, I'll recompress the file (video compression) and recompress using both rar and 7z again.