I was the same as you, I stuck with advanced trainers for a long time due to their salaries and the fact they train players at a good rate, however since changing to a superior trainer, I'll never buy an advanced again, despite there not being much of a difference. The amount of level ups totally depends on a number of factors:
a) age of trainee
b) potential of trainee
c) elastic effect on skills
d) what your training
For example, if you train an 18 year old MVP and train Driving for the whole season, you would expect more pops than if you trained an 18 year old allstar being trained in pressure the whole season.