Most commercial string trimmers spin counter clockwise...keeps the mess off the sidewalk and out of shrub/flower beds.
Nothing I hate worse than one of those cheesy curved shaft string trimmers that spins clockwise.
Those are good heads, but the only thing I don't like about them is if the line is brittle or the weather is cold, one side (or both sides) of the line will break off right at the exit holes on the head. Then it takes a screw driver to pry the head apart to fix the line. The spool types are better in this regard, because if the line breaks off you can just pop the spool off and rewind it.
You can't wind the spool types too tight...then line melts together and makes a real chore trying to feed line out. Too loose, and the line will come out and tangle and wad up in the head.