Well, the biggest drawback of Windsong is that it is complete RNG. You could (theoretically) proc 15 crit buffs and 0 haste buffs over the course of a 5 minute fight, and that would be nearly worthless for your survivability. Now, Disclaimer aside, I personally like the functionality of Windsong a lot better than Colossus.
IMO, Colossus just isn't big enough. I used it on a farm night a few weeks back just to get a realistic idea of it's number output on any given fight. On the logs it averaged around 1.7% or 1.8% of my healing taken, varying from fight to fight. On an almost 9 minute fight (Horridon and Durumu were both 8:56) it totaled for about 450k healing, which is less than 3/4 of my total HP.
I think what I like most about Windsong is that it gives you stats that (to a certain extent) still requires you to know how to play your character to be most useful. In other words, it's a more interesting proc. Colossus just gets thrown under SS, nothing to do about that. You can make use of Windsong procs though. Haste, Mastery, or even Crit. They also have their more "passive" benefits though, so it's not something you have to keep a close eye on for it to be of any use if you're preoccupied. You can take it or leave it, and still benefit.
If I recall correctly, purely from a mathematical standpoint, Colossus is considered best. Not 100% sure though.