As a few people have said, the most important part is to find a pair that work together

A perfect example is those two great Australian half back pairings of Wally Lewis & Alfie Langer and Peter Sterling & Brett Kenny

In the Lewis/Langer partnership, Lewis was the general who bossed the game while Langer had a more dynamic running game while in the Sterling/Kenny partnership it was the scrum half who was the general and the stand-off who was more off the cuff and dynamic. These two pairs complemented each other perfectly

(Of course Lewis could take the line on while Langer had a passing game etc but I am referring to the dominant role they performed)

The waters have got a bit muddier recently and the Cooper Cronk/Johnathan Thurston partnership for example doesn’t separate the roles quite so explicitly but the key thing is that all of the key skills and responsibilities are covered

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.