Power levels are essentially noisy points (i.e., points with additional variance). The recent developers commentary made it clear that power levels represent a unit's average value across all possible wargear combinations. Units that have more wargear options (Inquisitorial acolytes are the extreme here) have higher variance. The more of such units you include, or the larger the army you play with, the greater disparity you get between points and power levels.
Basically gw used a very common statistical technique called "binning" (see here: https://en.m.wikipedia.org/wiki/Data_binning). Each power level is thus a bin into which units with similar average values are assigned. That's why, despite the variance in average cost between units like 5 skitarii vanguard, 5 sisters, an eversor, or 4 acolytes, they are all power level four: because they fall within the point limits of the bin assigned to four power levels.
Tl:Dr - use points unless you're lazy or like adding variance to your game.