That can be a tough choice.
Even if you go for one H110 option, you may still need two power supplies. (If you're using enough cards to exceed the capacity of one H81 board, one PSU might not be enough, depending on the exact number and type of cards.)
Also consider the possibility of failures. With one rig, if the motherboard/CPU/RAM/etc. fails, all of your GPUs are down until you have a suitable replacement. On the other hand, running 2 rigs may create more points of failure (yielding greater chances of A component malfunctioning), although the two systems will be independent.
With recent cards & drivers, Windows 10 can support 8 AMD and 8 Nvidia cards. Most users have to do quite a bit of troubleshooting to make that happen, however.
Personally, I would use 2 H81 motherboards for two separate rigs.
Yes, good. You right about possibility of failure. Low chance have two rig down for part same time, but with one motherboard all mining will stop.
But if look in future, Ashrock H81 btc pro use DDR3 memory card, Ashrock H110 btc pro DDR4 memory card. May be better to buy moterboard with DDR4, I means with new technology? Or for mining no problem witch type of memory use?