Sigma v Pentax longer lenses
Personally I don't find the lack of zoom too much of an issue as I normally carry two cameras and use a shorter zoom lens on the second body.
Regards
David
Flickr
Nicola's Apartments, Kassiopi, Corfu
Some cameras, some lenses, some bits 'n' bobs

It generally handles things very well - unless the light is poor and the subjects are at distance and then it fails miserably.
Optically the DA*300 is a far superior lens but it is also twice the price and unless as David suggests, you carry two cameras - a little bit limiting at air shows where I often find I need to be shooting shorter than 300mm.
Carl
No matter how many lenses I have owned - I have always needed just one more

I did the Duxford airshow last year and had my DA*300 and Sigma 170-500mm.
Most of the time I used the DA*300 due to where I was positioned (outside the airfield) and 80% of the shots were taken with it. A lot were cropped due to the lack of reach ironically, I would have preferred to have my 1.4 converter with me, but I only bought it in March this year. The other ones were where the aircraft went over our heads and the Sigma or indeed the 55-300 would have been the better option.
Personally, and it all depends where you are at Airshows, go for the 300 first and accept the fact that some images will be too close (or creative as the case may be) and relish the IQ in the first place. Then consider a Sigma for flexibility for future. Then again, find a spot that you don't have to have the Sigma in the first place and enjoy knowing that the 300mm will give you what you need.

Some Cameras

Phil1022
Member
Surrey
I Would really appreciate any thoughts or experiences anyone has with both or either of these two lenses .
The Sigma 500's have been largely discounted due to portability /size issues.