Modern UI icons vs traditional icons

I need to choose icons for the UI of a Windows Store application, and am debating between the modern ones such as these (first result for images of "segoe mdl2 assets") and the traditional ones.

I realize that not all apps are the same and this might be dependent on different factors. I can't, however, seem to find what these factors are. So that's my question.

I hope this is not seen as an opinion based question (which, I know, are frowned upon here) since I'm not asking what would you see as the most important factors. I'm asking for different factors that would affect this decision, so I can make an informed decision with that information.