Michael,cannot speak from personal use(I use Canon),but on BF there are many users of the Sigma 50-500.I have seen some superb shots obtained from this lens.But if weight is a problem ,I do believe the Tamron could be lighter.I use a Tamron 28-300 for general use,and it is a great little lens.
also I have seen some superb images taken using the Tamron 200-500.If you have time to spare,why not pop over to
www.birdforum.net the sister site to WPF,and go to the gallery,type the lens into the search bar,and you will see shots taken by the lenses.