For Vegas, there is no question. I’ve always been told jpg is better for photos in general (and with good reason), but for some reason Vegas seems to really like png files over other types.
I couldn’t even try to load my images without crashing Vegas when they were in jpg. Converting to png hasn’t solved all of my problems, but everything works 95% of the time now. I can pan, zoom, crop, etc – you name it. It’s still just a little shaky when the resolution gets higher.