So you want a longer lens and can’t decide which one to get. There are several good options and one of them might be better for you in particular. What about resolution? Do you want to be at 600mm most of the time or just some of the time? Does it matter if the zoom and focus rings turn in Canon’s direction or Nikon’s? What about size, weight, and cost?
The Tamron 150-600 lens is a good option, as are the newer Sigma 150-600mm Contemporary and Sport lenses. (Don’t buy the older Sigma 150-600 which is inferior.)
The new Mark II version of Canon’s 100-400mm lens with a 1.4x teleconverter puts you in the same focal length range at 560mm. Without a teleconverter it is better optically than the Sigma and Tamron lenses from 100-400mm. With a teleconverter the optical quality of the Canon lens is equal to the Sigma and Tamron lenses out to 560mm. The Canon lens also has faster autofocus. The Canon lens is more expensive.
So what it best for you?
Before you plunk down your hard earned dollars, read this review. The review compares differences in features and it will give you resolution comparisons at various focal lengths and apertures. No lens wins at every aperture and focal length.
It would also be a great idea before you make your purchase if you rent your top choice or top two choices and try them out before you buy. Now matter how good the specs look in a review, there is nothing quite like using a lens on your camera to know if you will actually enjoy using it. My favorite rental place allows you to buy a rental item if you like it (with some rare exceptions), rather than return item, and your rental price is deducted from the purchase price. More information here. If you rent one lens, like it, and buy it, you are out nothing for the rental. If you rent two lenses to compare and pick your favorite to buy, you are only out the rental for the one you return – and you have the satisfaction of knowing for sure which lens you prefer since you tried them both.
As of the today (August 16, 2016) the Sigma 150-600 Contemporary rents for $52 for four days, the Tamron 150-600 rents for $53, The Canon 100-400 Mark II for $76, and the Sigma 150-600 Sport for $84.00.
Sigma and Tamron 150-600mm lenses reviewed with the Canon 100-400mm Mark II lens added to the comparison toward the end of the article.