Bill... I'm assuming you plan to use a tripod with either of these lenses, yes? I don't own them but I had the opportunity to use the 150-500 in the field (on a Nikon mount). While I was pleased with the results... one never forgets that it is a big, heavy lens. I agree with Peter - I consider this lens to be marginally hand-holdable.
Also, while I haven't personally been able to compare the results from the 50-500 "Bigma" with the 150-500 "Lil Bigma," I thought the smaller lens delivered well enough that I wasn't wanting the other lens. I would say if you can get the 150-500 for less and don't really need the 50-150 focal range on a lens like this (and the truth is, we all buy these lenses for the long end, right?) then I would go for the Lil Bigma.
Are you planning to buy new or used? These lenses are expensive. The 150-500 is $1000 new.
I would use it on a tripod, (I have a really good tripod and a Swiss Arca head etc.), - we have some nice areas near by for "birding" plus if I went far I would always carry it by car - i.e. no long hikes
I would not use the shorter end at all - I want to try to "fill the frame" as much as I can
I would use it in conjunction with my bins - i.e. spot the bird with my bins and then try to capture a shot with my D300
I have a Nikon 70/300mm VR tele - but the reach is just not long enough to be of much use, most of the time, around where I live - the little critters are just too smart
I am looking at used lens - but I read that there are some "bad copies" around - some have back focus problems and some are just not sharpe.......... my budget is up to £1,000, (US 1,500), for a lens ............ a good used scope with the usual "bits" also comes in at that price range.
I am tempted by the Nikon 80 - 400 - but it is twice the price
I have and am also considering a Swarovski 80HD scope to use with a compact, (say my Canon S95) ............ but as I have the D300 and the (new) scope set up will be up to 3 times the price of the Bigma ............ I feel that the Bigma is a sensible way to go.
Lots of "wild life" near where we live ......... plus we travel quite a lot by car into Spain and the UK
I am "going around in circles" reading the Bird Forums - re advice etc., etc., - so I reckon I should just take the plunge and get the latest Bigma with OS
(PS - I have a 45mm 200mm which I use with my G1 ......... but again, not enough reach to be useful, plus unless the light is good, IMHO it is a soft lens)
Thanks