Model localization: one big model vs two small models
When and how to choose 1 vs 2? Data exploration, understanding the problem, and testing the models. Make sure you use metrics, a cut-off level and test data that is appropriate for your business problem. 

Take for example, if there is a fundamental difference between US and CA customers. Then allowing the model to focus on the signal of each country, without dealing with the noise of the other country, may produce a better model for each. By doing data exploration (plot purchases by country over time and some other important features) you might convince yourself that it is worth the effort to test multiple models.

This is the art vs science of modeling.