Single or dual coil I always use 1.8 ohms. The only time I use anything different, it's in my HH.357. Those are all 2.0 ohms.
The 1.8 is just my personal preference, it seems to be a good middle ground. The 1.5 ohm is too warm for me, even at 3.3v, which is the lowest my variable voltage devices go. At 1.8 ohms, it seems best around 3.7 - 4.0v for most juices but I have the option to go lower if the juice calls for it.
Plus, at higher resistances, my battery drains faster due to needing a higher voltage to heat the coils.
Like anything else in vaping, what resistance coil you use is subjective, and everyone's experience with them is different.