I posted this in another thread, so if you all have already read it you can skip it:
I have, own, and shoot the 50-500 "BigMa". I call it "The Beast", it weighs in at 4 pounds and after you have shot it all day long you KNOW it has been on your shoulder.
Image quality is surprisingly good for a lens with that kind of zoom range (10X)... BUT... and this is a big but... you shouldn't expect to shoot it handheld at 500mm.
Unless you are shooting at HIGH shutter speeds (1000th of a second or higher) it has to be on a pod or stabilized (I use bean bags) when shooting in the 400-500mm range. This is one lens that desperately cries out for Image Stabilization, but doesn't have it.
Another quirk about the lens, and one that is quite annoying, is that it has a weird front filter size and no drop in filter tray... so you have to buy all new filters just for it and they are EXPENSIVE at that size. I generally carry a couple ND filters, a polarizer and keep a UV filter on all of my lenses, but a polarizer for the BigMa is $300 alone. I don't, therefore, own one.
The lens is fairly sharp wide open, which is good, because having to shoot at 1000th of a second pretty much means you are going to spend a lot of time wide open or nearly so.
Here is a shot I took with the Bigma at 500mm to give you an idea of what it can do (I posted this recently on another forum). This is at a zoo, where the bird was backlighted (hence the blown out top of the head), and I am shooting through a cage which (obviously) hurts the sharpness of the picture a bit... however, the lens is GREAT at blowing through cages. The picture isn't very good, but you can see that the lens is at least fairly sharp.
Here is a tiger (also shot through a cage at 500mm) with the lens wide open:
I have looked at the Tamron, held it, but not actually shot it, so I can't give you any advice on it. I have a firm rule of not giving advice on buying equipment that I have never used...
I don't really care about how well a lens shoots a test chart, because in all the years I have been shooting, I have never said "Wow, here I was walking through this lovely forest, and look what I have found... A TEST CHART hiding in the bushes!!!! Hurrah!" and I don't find it of any use to offer opinions on equipment I have never used... so I just don't do it.