I've been holding out on upgrading my last Sophos SG210 firewall until the new models came out. I'm torn between the MX85 and MX95. On paper, the MX85 looks sufficient with the exception of the 250 device count, although I've always been able to far exceed the device count on my MX67's with no problem. I'm curious how these stack up to the MX84/MX100.
As you know the client count is a recommendation based on mixed traffic types and all services enabled. So, in some situations you could exceed the recommended number and still be fine. I always advise to not cut corners when sizing and factor in your anticipated growth.
As for the new MX's vs. the older 84 & 100. You definitely want to go for the new platforms. Tons of advantages on the technical side (throughput and port types).