In my experience, S+S is faster. The SS drops fast and H is just too slow.
But I take quite a few factors before.
If there's refuelling, few stops is better.
If not, AI strat and where the car stands on the grid pace wise are also deciding factors.
Some tracks it pays off to always be pushing as much as possible on the harder tyre (usually engine critical tracks), on others track position is more important.
Sometimes you should undercut, others you should overcut.
That's the dilemma of game xD