Which Sigma Lens, Sigma 50-500 or 150-500 ? (1 Viewer)

Hello I am very new to birding photography , My kit is a Canon 450D and a Tokina 80 - 400 lens .
I was thinking of buying a new lens perhaps one of the Sigma 150 - 500 with image stabilization on but didn't know if it would be better for me ? and would this lense be better than the Tokina 80 - 400 ?
Also i have a Kenko AF 2X Teleplus MC7DG conversion lens for EOS Canon , Can anyone tell me if this converter would fit on to the Sigma lense and work correctly ?
If the Sigma lense would be a good bet can anyone give me an honest idea what the Tokina AT-X 840 AFII would be worth if i was to sell it ? It is in very good condition boxed, Please try and be honest .
Many Thanks, Geoff.

I've got a Sigma 50-500 but it's an easier choice for me because my camera has a very effective image stabilisation system built into its body. I was still tempted by the 150-500 as it's a bit lighter and shorter but I want to be able to throttle right back for my other passion of motor sports where the subject gets very big very fast and I want the widest possible zoom range. I think that if I were solely shooting wildlife I'd have gone for the 150-500. It's down to what you want to shoot, but both are good lenses.


Any teleconvertor that has the sensor links should work with any compatible lens, but you're unlikely to get AF with a super tele lens. All your other communication with the lens should work, however, such as aperture, focal length etc.

Sigma lenses are a good and economical solution to upgrading your lens package and of a high quality.

My only advice on a likely price for your old lens is to stake out eBay for a bit and see what they go for.

Thank you for the info