Rinnegan is the ultimate form of the sharingan.
rinnegan> EMS > MS
- if you are a true rinnegan user you are also a EMS user,they are not exclusive.
Rinnegan comes with control over gravity, human souls, life and death, + a EMS.
So, a "true" rinnegan always trumps a EMS. Because it includes a EMS within its powerset.
A sharingan is just a diluted rinnegan.
Now that we are clear on the rinnegan/EMS discussion. let's compare rinne sharingan and six paths senjustu.
First, let's be specific. The op asks which is more useful,- to answer we need a context. I am going to assume the op means more useful in everyday life for me personally, and not what's more useful for a ninja. Furthermore, I am going to assume both abilities are fully mastered- and used at their utmost potential.
Six Path Sage Mode:- a combination of sage energy and chakra of the biju.
a complete six path sage mode ( all biju) gives the user incredible physical strength and speed. Flight, and regenerative capabilities.
And the gudodama. Making the user near immortal.
The best part of the six paths sage mode is that it comes with a state of being that allows for a instinctual understanding of the universe, this explains why madara was able to use storm release and whey it was easy for naruto to combine biju chakra with his rasenshuriken.
- being near immortal + with a ability to grasp anything ( in the universe) with the skill of a well practiced master is pretty amazing and extremely usefull. -
I'd say that trumps the sharinegan quite a bit. -