The sony 70-400mm f4-5.6 is very similar in performance to the nikon and canon offerings, autofocus isn't lightning quick, but image quality is adequate, the only real optical weakness of the lens is the quality of the bokeh (out-of-focus blur) at the long end of the zoom range. On the expensive side compared to other 3rd party offerings, around £1400/1600.
The tamron 200-500mm f5-6.3 is a usable lens, but definitely needs support, tripod or monopod, its not fast either and need good light to achieve good images due to poor performance in low light, its an affordable offering at around £900.
It really depends on your budget, but to get anything decent, your talking over £1000.