Well the 100-300 is 52.2 oz (1.5Kg), and the 50-500 is 64.9 oz (1.8Kg). That is not that great of a difference in weight.
In my neck of the woods, the Bigma is around 100 cheaper.
I am not including the cropping factor in this example. If you use a 2x teleconverter on the 100-300 f/4, you can get a 200-600mm f/8 lens (you lose two stops of light). If you use a 1.4x teleconverter, you can get a 140-420mm f/5.6 lens (you only lose one stop of light).
The Bigma can get you to 500mm with no need for a teleconverter at f/6.3. Plus you can get a slow but equally handy 50mm f/4 at the low end.
I have the Bigma and like it quite a lot (Canon Mount). I can see going the other way if the lens was an f/2.8, but then it would be more expensive, and a great deal heavier then these other two lenses.
Basically its up to you to decide which lens you plan on using more often. If you are planning on using 400-500mm more, then I would suggest getting the Bigma. This way, you don't have to fiddle with an extra teleconverter. But that is just my opinion. On the other hand, you can get an f/5.6 420mm. It all depends on what your planning to use this for. Either or. Sorry I couldn't be more help to you.