They are both in the small planet-moving or planet-busting range, so they are probably close strength-vise.
However, during the official JLA/Avengers crossover, Superman was able to stop Thor's hammer with a single arm (Something I have only seen the Hulk do before), and with great difficulty overpower Thor, despite less skill, and not using his super-speed. So I would give superman a certain edge.
Of course, Thor can increase his strength 20 times by donning his belt of strength and entering warrior's madness, which would give him a great edge in terms of strength alone.
On the other hand, Superman could likevise fly into the Sun and power up, but I don't know if it would increase his strength as much as 20 times.