It's hard to have precise numbers to compare Windsong vs Jade Spirit, because WoL doesn't tell you when you got the spirit buff.
From Derevka's disc guide on HowtoPriest.com :
By using Windsong, you gain about 900 of a mix of throughput stat, compared to the 396 intel from Jade Spirit. Plus you can count on more regen as you got an average of 300 crit vs 45 spirit (depending on your spell usage I consider 1 spi gives as much regen as 1 to 4 crit, so 300 crit > ). The thing is, you'll never know if Jade Spirit will proc while your under 25% mana or not. And, you could very well spend less than 25% of the fight under 25% mana. Personnaly, I tend to use Mana Tide as soon as possible, and I'm rarely under 25% mana until the last burning phase of the fight, so I might not even need that extra spirit if it comes too late in the fight. So in my opinion Windsong gives you more reliable regen, with about as much throughput as Jade Spirit.According to final clarity from Blizzard, Jade Spirit equalized, should provide 396 intellect, and additionally has a 750 spirit proc when you proc the spell under 25% mana. (which results in ~45 spirit equalized, assuming you are under 25% mana for 25% of the fight). This is using the 12 second buff duration, and 50 second ICD and assuming you proc it as soon as it comes off CD.
Windsong is a bit trickier to evaluate since it can provide three possible buffs: 1500 of either Crit, Mastery, or Haste. Each has its own ICD, and multiple buffs can be active on a player at one time. It has a 1PPM, with a 1 sec CD between each individual stat proc. This puts each stat with ~20% uptime, which means the enchant is roughly worth 300 of each Mastery, Crit, and Haste
(And this is true no matter how much spirit you have, but I can understand why someone wanting as much spirit as possible could prefer Jade Spirit over Windsong)