not sure which ones are selected for having the option to choose this
It is only available on devices that have the ROM space for it without repartitioning. The N7 2012 and N10 do not have the space for ART. That is also why other N10 ROMS do have it because they can exclude stuff that Google includes.
Don't think it's the space: My N7 2012 and N10 are both 32GByte devices with a unified file system layout so there is tons of space.
I've seen comments that indicated that ART was tuned for Qualcom CPUs and wouldn't deliver performance advantages on Tegra3 (N7 2012) or Exynos (N10).
OmniROM has it available on both but I've never bothered to compare and it may actually be pretty difficult to do.
It is only available on devices that have the ROM space for it without repartitioning. The N7 2012 and N10 do not have the space for ART. That is also why other N10 ROMS do have it because they can exclude stuff that Google includes.
Does it means that the N10 will not have the 4.5/5 update who is supposed to be ART only?
not sure which ones are selected for having the option to choose this