Reason I like to use a vardiff port is to try and get a share within a few seconds.  I prefer to have my rig run about double to triple diff to hash. So if I have a 100 sol/s rig my diff would be around 200-300.  That way when I have a block change and shares are rejected I don't loose as much work. I'd rather a 300 diff share be rejected vs 6400.  That is my reasoning. To each their own. It really boils down to how your rig performs on each port. Everyone will have different results if only perspective.

BR