Upgrading some old switches to latest FOS. Which FOS?

Hi friends,

I have 4 Brocade 4900 and 2 slightly newer 5100 switches connected using FOS 6.4.2 and I'm trying to stretch their use a bit by upgrading them to the latest possible FOS.

I found out that the 4900 supports FOS 6.4.3d and the 5100 supports the 7.1.1 version.

Does anyone have any experience or advice to what FOS to upgrade? I could go to version 6.4.3d, so each of my 6 switches runs the same version, or I could go even further with the two 5100 switches, but I would be running a mixed FOS 6 and 7 environment.

Recommendations anyone?