A side pull is a nice tool to use when starting a young horse or schooling an older horse. For the young horse they are great b/c there is a nice progression. You can go from a bitless Side pull to one with a bit to a snaffle to a shanked bit.
If you start a youngster in one, I would suggest not using one with a rawhide noseband. They are very hard and put painful pressure on the nose. An english style sidepull has a padded noseband that is much gentler.
The rawhide one may be better for an older horse that is needing "remedial" work, IMO.