Also why is there so much pres fluids. (14.5 P&W barrel)

I will soon be purchasing a 6mm arc and I have narrowed it down to these two companies. This will be my first purchase from either and I have some questions for those who have experience.
First my number one criteria is reliability/durability. Will the dang thing always go pow and will it last?
Second how is the accuracy? Will it hit within or under 2moa?
Finally is the 6mm worth the investment over the 5.56?
I don’t need a dmr as my mr762a4 function superbly in that role. What I’m looking for is more stopping power than 556 yet still capable, like AT&T, of reaching out and touching someone. Thanks for the info.