Hie thee to a fiber festival and try out everything you can get your hands on!
Seriously, that's the best way to figure out what will work best for you. There are many choices, depending on the type of fiber you like to spin best, the amount of strength you have in your hands and wrists, the type of prep area you can devise, and the portability you require.
Major areas to look over: be sure the individual steel tines are set firmly into hardwood; make sure there are no rough edges on wood or metal to catch your fiber; try to get a set with a tine straightener included; don't forget (if storage isn't part of the package) that you'll need to store these safely.
When to use? Anytime you want a worsted preparation - for a worsted yarn!