In an overall Windows user experience? Definitely moving windows between screens. Selecting tabs is a pretty niche and uncommon feature across the OS.

It's not about which action is more common, its about the fact that the titlebar drag gesture is universal cross windows 7/8 and is a key element of the user experience. There are exactly two apps I've used in the entire history of windows 7 that broke this interaction: Chrome and FF. The fault is on those browsers breaking expected behavior, not Opera for doing what windows users expect. The FF UX team admitted that they overlooked this but its understandable that they're keeping it as users have come to expect it.