I'd suggest moving to R80.20 on both, management and the gateways..
In terms of VSX, you'll gain the 64 bit support for VS' that will allow better memory allocation.
If your client is looking to implement HTTPS inspection, this may come in handy.
Additionally, there is a much better implementation of the SaaS services objects, such as Office 365 as well as FQDN Domain objects.
I'd stay away from the M train, unless you have a solid justification for taking that route.
After M1, upgrade to GA required involvement of TAC for quite a while, although it may have changed since.
Staying on R77.30 on gateways longer than necessary will limit your available functionality and as a result you'll be using temporary workarounds instead of taking advantage of latest features.
In my experience, there are few things more permanent than "temporary solutions".
This being said, the R80.30, now in EA, adding quite a few new capabilities and addressing few shortcomings of previous releases, at least on paper.
If you are not in a particular hurry, I'd lab the EA and roll with it once it is in GA.