R5E.10 would be the even trade (essentially) for the R5E or R5E-3.1, withe the x99 Strix being a 'downgrade', but is still a board that meets all but the 1%'s requirements typically.
If you're a very very very hardcore tweaker and use the R5E's extreme detail OC settings, or use L2N and love the bench features, (or need the dual LAN on the v10 and don't have the room for it in your PCIe plan) then you'd likely want to stick with the R5E line......if not, then the x99 Strix is great-great.
In all honesty though, I like the significantly more back-panel USB 3.0's of the R5E-3.1 and just added a dual type-c front panel 3.5" bay to add that functionality.