My guild is looking at merging our 10-man heroic raiding teams, of which we have 2, and recruiting a few others to start 25man before the change to Mythic in 6.0/WoD. At this stage it looks like I will be staying BrM in this transition, and wanted some advice on what best strategy to go for gearing wise when from 10 to 25.
I do currently have in my possession a Heroic tier chest token and heroic tier glove token and a normal tier essence from Garrosh to be able to switch and play around with set pieces. I currently don't use 4-piece because on 10-man logs I never saw the heal doing anywhere near as much as I had hoped it would, but will be using the 4-piece in 25-man due to the higher damage/more purifying required.
The 4pc can be very useful. In terms of gearing it depends on other factors. If you healers are good then you can continue with the full crit build. If you want to play it safe then switch from Crit to a more mastery focused build to stagger more damage. After you get used to the new damage with more mastery you can start scaling it down and back into Crit.