I'm betting the S6 Edge demand exceeds all expectations by a very big margin. Everyone I know, and I mean EVERYONE, that has looked at the demos has gone away saying they will be getting the Edge hands down. In fact Samsung is said to be ramping up production on the S6 Edge because increased demand by partners. We shall see...
Saying you would prefer the edge hands down and parting with the extra $$ 150-200 per model for it is a different thing indeed however.
Likewise demand will likely always be higher than supply simply because Samsung have said themselves that supply constraints will be a factor due to the fabrication process / constraints of a curved display.
I still think whilst people would prefer to have the Edge - the extra cost for nothing really more than cosmetic difference will make the 6 still the far bigger seller to the same level as iPhone 6 & 6+ sales differences (3/1)