I have both Yammie clears, the stock and the tall screen. Here is what I've found:
Stock - @ 5'10", the wind hits me in the chest and the helmet is in the wide open air. There is more wind noise because of this and less head buffeting.
Tall - Wind hits just above my visor and rolls over the top of the helmet. The tall screen creates more of a vacuum behind it. Because of this, it's slightly quieter but the head buffeting is worse. It certainly deflects more wind, bugs and debris though. Tall also acts as a windsail. With higher winds, the tall screen tends to push the bike around a lot more so if it is windy, I try to stay out of open spaces and stick to wooded back roads where there is wind protection.
Conclusion: Both have pros and cons. Best to keep both around and use the stock for twisties, shorter rides and around town stuff, especially if you live in a place where it tends to be more windy. Put on the tall screen for long trips only where you want deflection of wind, bugs and debris as well as added rain protection should you get caught on the road.
I'm going to mod my stock screen with a custom paint job on the inside after hand painting a cool design I came up with.
As for the tall, I'm thinking maybe it should be vented at the bottom in some fashion to reduce the vacuum behind it. This is taking a lesson from the '07 & '08 FZ6 design.
Hope that helps in some fashion!