Here are the personal downsides for the current Z170 and I am going to assume Z270 motherboards.   The majority of them only really support 2 way SLI and only a handful support 3 way and 4 way.    Now many will chime in and say "Well the the 100 series only supports 2 way SLI"   True enough, but I don't have a 100 series card and I don't plan on buying one (or two, or three) any time soon. So I need a motherboard that will support up to 4 way SLI.  Had I jumped on this bus a year ago, this wouldn't have been an issue as the Classified 4 way would've still been readily available.  
 
And a minor gripe, the vast majority of the "performance/overclock" boards are either ATX or smaller.  I prefer EATX.  Weird, but I like big boards.  (Cue Sir Mix-A-Lot music)
 
Now that the EVGA X99 Classified is available again, it's a choice between that and the MSI Xpower Titanium.   That I think is a tough choice.  The EVGA is less money and their customer service is second to none.  But I've yet to have a bad experience with MSI motherboards.  And to be fair, I have yet to have a bad experience with EVGA motherboards.
 
So, Im going to kick this over to the X99 forum now.