Predict which user will buy with an offer - discount
You can use Decision trees for a single model prediction for both the set of users.

A good start would be to first read up on Decision Trees and their applications.

You can include the offer as a decision(as a boolean in this case). 

buying with offer and buying without offer can be the decision criterion.

You can, in fact go ahead and put in the offer values also. For example, 

offer&gt;10% and offer &lt;10%