For maximum rig density nothing can beat Linux distros because they can handle those new boards with 19 available slots. ethos and smos are very popular but may have monthly fees per rig. Windows 10 has resource issues with some boards and can't do the insane density linux can.

If you're going to be using AMD cards then Windows is easier to flash BIOS and control voltages. Windows has the pain that is forced Windows updates - not a problem for those who know how to shut it down. Windows is more stable with mixed AMD/nVidia rigs. Windows tends to mine a tad bit faster than linux distros.

As far as uptime, they're the same. I've had rigs some stay up for over 6 months straight and only come down because of a power outage or cleaning.

Most would say pick the OS you're most comfortable with.