Thanks so much for the links and the advice. I am still reading up on both and learning more.If you want to buy the lens, and I will recommend to read more reviews from online, because either of them is not cheap.
Tamron 100-400 https://www.the-digital-picture.com/Reviews/Tamron-100-400mm-f-4.5-6.3-Di-VC-USD-Lens.aspx
Sigma 100-400 https://www.the-digital-picture.com/Reviews/Sigma-100-400mm-f-5-6.3-DG-OS-HSM-Contemporary-Lens.aspx
From what I read the sigma does not have tripod ring, and for Tamron, you can buy one, but from the review it will be expensive. Because the lens without the tripod ring, then you will find the limitation to use the lens in low-light conditions, even the lens has the VC or OS. And you may say I can use the tripod with the camera body, but for the 400mm end, you may not get the share pictures.
By the way you may consider to buy an extra battery for your camera too, if you don't have one yet.
One more, it will be the same: read more reviews before you buy a lens. And always buy the best lens you can afford, because you can use it for long long time, and you can find the good lens will have more resale value too. If you just buy the lens, and later you find out you don't like it, and you will spend more money.