I have a customer running ISE 2.4 looking to upgrade. The current ISE golden image version is 2.7. But 3.0 is also available and it seems this will be the long term build going forward. I understanding the licensing implications of 3.0.
I might go with 3.0 for a brand new build, but I would still go with 2.7 for any production upgrades. It really comes down to the risk tolerance, if there isn't an immediate need for 3.0 features, then I would play it safe.