superzoom advice (1 Viewer)

I currently have a Sigma 70-200mm f2.8 which I use with my Nikon D300 but I am hoping to get a longer lens to help me get some better images of birds and other wildlife. I could use some advice.

I have been looking at the Tamron and Sigma 150-600mm lenses but £700 is going to be a stretch for me at the moment for what is essentially just a hobby. I have seen some Sigma 150-500mm lenses for around £300 on Ebay and I could potentially afford these (Christmas and Birthday both coming up!).

My question is should I buy the Sigma 150-500mm or wait and try to save up enough money for the 150-600mm? Apart from the extra focal length will image quality be noticeably better on the 150-600mm?

If you have owned or used these lenses I'd be very interested in your opinions.

I owned and used a Sigma 50-500mm lens on my Nikon D300 for quite some time. I wont say its exceptionally sharp at full 500, but nor are any zooms at the top end. I was quite happy with it, that was until I bought the Sigma 500mm prime lens, and I could see the huge difference. I have quite a few images on Birdforum taken with the old combination, but you may have to go back almost 3 years to view them.

Save for the Sigma 150-600 C it takes some beating without spending loads of money,the D300 is a great camera i did have one but its not the best for cropping so the longest lens you can get is the best.
Dont want to upset any one but i dont think the 150-500 is a top lens.