Shunyata Sigma Analog vs Alpha Analog - Need advice!!

I have a chance to buy these PC for my ARC Ref 6 now:

Shunyata Sigma Analog 20A (100% brand new) = USD 1800

Shunyata Alpha Analog 20A (2nd hand) = USD 645

I have never compare these cables side by side or listen to it before, but I heard about many good things about Shunyata , especially how good it is to work with ARC amp. Since Sigma Analog are much more expensive than Alpha Analog, does anybody has experience to compare with them?  Does  Sigma Analog much better than  Alpha Analog and worth to take it at $1800 ? This amount is expensive  for me, I need to make my decision very careful.                                         

PS:  I'm using a Furutech DPS-4 PC for my Ref 6 now (with Shunyata 15A to 20A IEC converter) , I'm afraid if I choose  Alpha Analog but the sound quality is much different with my  Furutech then I will be regret why not to take Sigma  Analog....Anybody can give me some advice or suggest? THX