very little is the truthful answer. one small difference is that 7 is more likely to perform the dummy half role if the hooker is not there/tackled etc, and more likely to be able to swap into the hooker role - ie: abrasive, able to scoot, decent pass off the deck etc, as well as being the playmaker on 1 side. The 6 generally needs a bit more of a kicking game, and may be a bit more fragile defensively (benji). if a player is more of a kicking pivot type playmaker, he would probably play 6, and sometimes move into 7 (sean johnson, stacey jones sean long danny mcguire etc) whereas if he is more of a low-centre of gravity quick-footed agile wee ball player he would probably gravitate towards 9 with the ability to cover 7 too.... thomas leuleai, michael monaghan, rob burrow etc.

This is just my take on it - i have no issues being shot down if somebody wants to add a bit more detail.

A scrum half is more likely to cause a fight.

Great point it always seems to be a scrum half who starts fights.
(Also hookers do obviously)

One Usually has 6 on his back and the other has 7.

I guess it depends what the coach wants from their halves. Sydney Roosters play with Maloney on the left, Pearce on the right, and they share the play-making responsibility. North Queensland Cowboys have Thurston who basically takes both roles upon himself and his halfback partner Robert Lui just plays off him. Canterbury Bulldogs use their half-backs as second/third receivers and let their prop forwards take first receiver. It really does vary with every team. The definition of a stand off or scrum half is really want the coach wants from them.

I prefer the title 5/8th.