Anyone here used the Sigma 120-400MM or 150-500MM??

[CAT-THE-FIFTH]

Currently on an older Sony system with the screw driven Sigma 135-400MM lens. I intend to move over to a Pentax or Nikon system at a later date,so I was wondering what people think of the 120-400MM or the 150-500MM?? Will be using it for general birding but also the occasional airshow - are the focus motors up to the task??

Any alternatives under the £800 mark??

[Bobster]

http://www.ebay.co.uk/ctg/Sigma-EX-5...Filters_Lenses