Always check reviews when deciding to buy a lens. There are so many factors beyond what is written in the name.
Note that in general, comparing between mounts is not a good idea. (Even comparing lenses on a different shared mount than your own is a bit better.)
(For many of these, use google search, as they are forum based.)
This is a "community wiki" post, where answers do not have individual ownership. Please use the answers to rate and comment on each site.