I own and regularly use the Sigma 50-500mm and I can highly recommend it. Also, contrary to what George says I've never even touched a tripod
I shoot aviation and it's a very highly regarded lens in that field.
I don't have any experience of the other lenses I'm afraid but I can give you a brief rundown of the 50-500mm
Things I like about it:
1) It's got great reach. 500 x 1.5 = 750mm
2) It has very good optical performance
3) It's got very fast HSM focus. My 70-300G sometimes used to hunt all over the place. Not so with this baby.
4) It's a fantastic price for what you get...probably the best value superzoom available
5) You get a lot of attention when you use it
Things I don't like about it:
1) It's heavy. I hand hold this lens, and I'm far from being the biggest guy in the world, but there's no need to go to the gym after a day's shooting with this. Still, I think most lenses in this category are fairly heavy.
2) It's not the fastest of lenses. 6.3 at 500mm means that for hand holding I either need good light or I have to bump up the ISO. However, I haven't found it really problematic.
3) the zoom ring takes some moving to get from 50mm to 500mm, and it's a little stiff on mine so quick changes of focal distace can get a little awkward sometimes, again, in reality I haven't found it to be a huge problem.
4) You'll need a big bag to carry it around.
5) You get a lot of attention when you use it.