The lower output smoke units seem to last well as long as you don't run them dry forever.
But the high output ones, that are fan driven, and especially if you are giving the heating element close to 1 amp, you have to choose your fluid, or the wick clogs and burns pretty darn quickly.
simpler ones that are basically a nichrome wire coil immersed in fluid don't have this issue.
So the "best" fluid can depend heavily on the type of unit and how hard you run it.
Greg