Zoom camera lenses for birding and nature?

I want to buy a zoom lens for birds etc… However money is tight, so can only afford lower end lens. I would like expert opinions, plus people have used both and give me your thoughts. I'm tossing up between a ( Sigma 120-400mm f/4.5-5.6 APO DG OS HSM for Nikon + 1.4x Tele Converter EX) or the (Sigma 150-500mm F5-6.3 APO DG OS HSM f/5-6.3). Let me know your thoughts.

The Sigma 150-500mm comes highly recommended for the price on Fred Miranda. Not much info is out there review-wise for the 120-400mm. If you are considering using a teleconverter on these lenses, they are too slow and I highly doubt they will be compatible. In order to use a teleconverter, you need to use top quality, fast, Nikon products such as the 70-200 f/2.8

The birders I know that are on a budget have either the Sigma 150-500mm or the Bigma (Sigma 50-500mm).

In reality, optically there's not a lot of difference between 400 and 500mm, at least not enough to put a 1.4x teleconverter on the lens. You pay a huge penalty in light reduction for not a lot of increase in focal length by using a teleconverter.

The price difference between the 120-400 and 150-500 is I think less than the cost of the 120-400 plus teleconverter.

Go to a camera store and have them show you a 150-500. Then look at it at 400mm and then 500mm. You will see that there's not much difference. I would be satisfied with the 120-400mm, but I'm not a birder, and they like the 150-500mm because they want everything they can eek out of the lens.