It's really hard to say. No one in lvl 90 will ever reach the amount of haste, crit and mastery you will be having by wearing MoP gear in level 80. Not only that, but you will be having acess to less abilities, and your base damage of each spell will have a different value.
My only suggestion would be to do the testing and simulation yourself, if possible.
But I prefer to use windsong as you can get a haste crit and mastery proc all after each other, had it a few times, and more than once each per 45 seconds, if they all pop I found the dps better than one jade spirit proc every 45 secs
Math can be close pre/post Haste cap, but Jade Spirit will edge out Windsong. However the difference in mats is enormous and you'll want to wait to shell out for Jade Spirit until you get a good Epic weapon drop.