I have surveyed more than 300 planets in HighSec and LowSec to date, and have recently been surveying the WSpace's that have been spawning out of the Provi area.
Although there are some obvious increases in resource yields when you dive deeper into LowSec, the increase is nothing when compared to the increase in resource yields you see in most WH systems.
For the more abundant resources, there is certainly no lack of supply in LowSec or WSpace. So talking specifically about the rarest resources (Felsic Magma, Reactive Gas, and Autotrophs), of the last 10 or so WH systems I have surveyed, at least 6 of them have seen amazing yields for these rare resources. I have begun measuring resource availability on a rough percentage scale when viewing Planet Mode in the scan tab, where you are looking at the horizontal resource bars. Most High and LowSec Lava planets, for example, show Felsic Magma in the 0-20% range. The last 4 WSpace lava planets I have surveyed have consistently had in excess of 60%. We actually had a Class 3 Wspace open this week that had every planet, including 2 temperate and 2 Lava. This system had Felsic, Reactive, and Autotrophs over 75%.
If not for the fact that the C3 system had only a single static to NullSec, it would have been the most ideal PI system I have seen to date.
Bottom Line: it clearly looks to me like WSpace is the best location for PI. You do not even need a POS. Drop in a scanning Alt and a Hauler Alt, either one skilled for PI, and you can configure your resource chains for maintenance every 2-3 days.