Sigma 120-400 or Canon 100-400?

G'day mates

Just a quick post to ask your opinion bout both lenses, to work with a 7D.
I have tried a research on this forum and seems like a very few use the Sigma. How come?

I have heard many things bout the Sigma, seems like first series got some issues but everything is fine now.
To be honest, I tend to go for the Sigma at the moment due to the price and I dont like the push/pull thing on the Canon (as well as the white color but can be easily fixed).

In both case, I will try the lense before to buy it in order to be sure their is no issues.

I have been considering prime lenses; as I use primes for my other Photography field; and will probly go for them later.
Indeed, being new into Wildlife Photography I dont know yet what range Im gonna use the most.
If I end up with many pics at 400mm...I will get a 400mm prime or more. But for the moment, I have to find my way and prefer the versatility of a zoom.

And as a complementary information, Im focused on Birds at the moment, mainly at the mangroves.

Thx guys

Andy