get the ones with the straight perches.
sparrows are able to use the ones with T-shaped or loop perches, but not the straight ones. maybe they can't turn their head to the side to feed.
GOOD

BAD

the socks and mesh-type ones are cool in that you are not restricted by the number of perches, and it's cool to see the birds hang upside-down, but their "open" design means trouble during wet weather.