I am a bit of a traditionalist, but a standard user flows is boxes and arrows with no UI info. It helps you decide how things need to flow together. the one with screens is now being called a wireflow, not a user flow. If you know the flow already, I would do a wireflow to help the clients. If you are still figuring out the flow of the app, I would do a standard User Flow.
Hope that helps!
Oh, and there are many, many software options. You can use Sketch for either option. there is a new tool called Overflow for wireflows. Lucid chart is also a good option.