mobo opinions?

I'd get a Gigabyte over EVGA, and an ASUS over either. The reason comes down to firmware. While all companies use a basic UEFI written by American Megatrends (they are basically the only company that makes one) they all customize it and work on features that can affect system stability/performance. ASUS seems to do the best, Gigabyte and MSI do very well, and everyone else is somewhere behind that. EVGA doesn't tend to be that great with motherboards, GPUs is what they are known for.
 
Also hardware quality is something of an issue. ASUS, Gigabyte and MSI all tend to be top notch with their motherboard hardware (within price constraints, of course, better boards have better hardware). I haven't looked at EVGA to see how there's is. Probably good, their graphics cards are, but it is a consideration.
 
Now that said, ASUS has absolutely garbage support. If I have an ASUS product die on me, I write it off and just buy a new one their support is that bad. Their products are quality, they rarely die, but their support if you need it is atrocious. Gigabyte is OK from what I hear, no experience. EVGA is pretty decent, particularly if you wish to pay them for their cross-ship support (extra fee you pay on registration, kinda like insurance). If you do get ASUS getting it from somewhere like Amazon that'll take a return isn't a bad idea.
 
When it was my money, ASUS was what I chose. That's motherboards only though, everything else I get from someone else and the firmware was what tipped my choice. I seriously looked at Gigabyte though (they are who I have my GPU from). EVGA I wrote off because their reputation for firmware isn't great.
 
With regards to SATA connectors, do note that only a certain number of those are connected directly to the Intel Z170 chip, 6 to be specific. Any extras will be on a secondary SATA addon chip which may not perform as well (can't say specifically without testing). Also note that the Z170 doesn't support USB 3.1, so any 3.1 is via an addon chip as well, only the 2.0 and 3.0 ports are on the Intel chip.
 
M.2 connectors, at least when used with a PCIe SSD, always share bandwidth with PCIe connectors. I mean in theory they could have a dedicated link, but in reality there aren't enough PCIe links on the board, so they'll share with a slot. They only way they share bandwidth is over the DMI link to the processor. If the motherboard maker hooks the M.2 slot(s) to the Z170 chip, which almost all will (the PCIe on the processor itself is usually reserved for graphics cards) then it'll share the 4GB/sec DMI 3 bandwidth from the Z170 to the CPU with everything else on the Z170. That's plenty though, and the "sharing" really won't be an issue.