Saturation model

I am very new to pymc. I am currently modelling order behavior for product families to derive the long term behavior. I used a linear model where I transformed the actual numbers into logspace as my assumption is that most of the product families will grow over time exponentially. However some product familes don’t grow any more but are in saturation (that is either the market doesn’t grow or competition prohibts a further growth).
I have two modelling questions:
a) is there an easy way to model saturation behavior
b) It would be great to run the same data on both models (the linear growth model a well as the saturation model) and get a comparison measure, how could I do it?