Positivist or Interpretivist approach?

Research Question:
I am trying to understand the Project management success factors in the maintenance (or after go-live) phase of the ERP implementation.

Philosophical Approach:
I am still having trouble about selecting the right approach for my ERP study. Looks like Markus et. al. is totally against an interpretivist approach in evaluating ERP success in their article called "Learning from Adopters' Experiences with ERP: Problems Encountered and Success Achieved"

Some Issues:
Also some of the problems I am facing are related to defining and measuring ERP success.
Actually maybe defining success isn't difficult, I have a good idea about it from the literature. The problem is how to measure it really.

Another issue I am having is how to select the sample. Ideally one wants to select successful companies or companies who have project success. How to identify those is a real problem when success now isn't really a predictor of success later.

I Just Though Of Another Question:
So is project management success in ERP implementations different from ERP Success? Okay, so I need to define both project management success and ERP Success. Actually ERP success and ERP Implementation Success are different things. Geez..

Okay, regardless of the philosophical approach, let me think about the research method...Maybe, I need to have a longitudinal study. So if I take some companies and identify some success factors and project management related factors... And then measure them across time. It might work.

Okay, what is the downside of this method?
1) Well, all companies need to be still within the same phase during both measurements. Because different phases have different success factors. So if one company slips into another phase, then I will be in trouble.
2) What is really the d