As a suggestion, don`t worry too much about users. Worry about User Throughput.
Layers with more BW should carry more users, consume more PRBs (absolute numbers), and carry more traffic. In your example, 100PRBs+50PRBs, under the fair division, you should see 2/3 of users and/or traffic on the layer with 20MHz. BUT, your spectral efficiency (a.k.a. CQI) is probably not the same on both, even your coverage may be different (PA Power, pA/pB, etc…)… So you may have 2/3 of users, but not the equivalent amount of traffic or PRB Load.
As long as User Throughput is equalized, you can use a single metric to compare both. As equalized, I mean, equal under a certain margin +/-5% or whatever you define.
That`s my 2c.