SHGC vs U-factor

The short answer is - it depends! There is no simple answer that will apply in all situations. Glazing selection also affects daylighting so visible light transmission (VLT) can also be important. Factors such as building type, climate, orientation, etc. all influence which parameter is more important. So is your building heating load or cooling load dominant, is it internal or external load dominant, what is the climate, is there any external shading, etc. are the kind so questions to ask yourself when exploring options. If you look at the building envelope tables in 90.1 you can get a sense of how these values change with climate. In hot places SHGC is lower and U-value is higher than in cold places. But this would just be a start. The purpose of energy modeling is to inform and guide design decisions. Evaluating these parameters in your energy model is very easy. It takes very little time to model a variety of U-value and SHGC scenarios and then simply compare the results. This is the best way to know which is more important in your situation. The calculations are software specific. You would need to dive into the code used to create the software in most cases to find out.