"300mm if birds are at all on the menu"
What kind of shooting tool are you using Paul? Is this dinner or a picture??
he 100-300 is a good choice and is faster without a TC but once you add a TC you have a lens that shorter (420mm w/1.4) and aout the same speed. Then there is going to be a little IQ loss no matter what the quality of the TC is.
Adding a 2x TC to the Sigma or other 70-200 f2.8 will also slow it to f5.6 and still not give you the same reach (400mm) with the resulting loss in IQ and problematic AF as many have reported with the 2x TC's and lower light situations.
So it boils down to a prime or the Bigma IMO. Pluses for both. 300mm will offer better IQ when used alone and very good with a 1.4TC but similar speed. But it's shorter and it's fixed. So if you want to do street shooting, you are stuck with the single length and that might pose some limits.
The Bigma is of course a compromise but every lens choice is, fixed or zoom. It's big and heavy but very hand holdable with the K10D and SR. I've shot great results at 1/60th near sunset with that lens. The focal range can't be beat. It' a bit slow and a little soft at the extreme ends but that's typical for a zoom from anybody. The main thing is the the weight. Both choices are going to be heavy for a day of walking without question. But the Bigma earns it's name here. the lens will get tiring after a period and prepare to get a backpack or sling bag for a more comfortable day of shooting. Last, some question the sharpness but I don't agree.
Have a look at this sample shot at a full 500mm with the Bigma f9, 1/400th ISO 100