What's better, one conductor or two conductors for an RCA interconnect?

The second standard is Differential input RCA which is balanced like XLR and uses two wires to send signal on wire one and inverted signal on wire two. No shield is used.

There is no standard for balanced differential amplification using an RCA connection. If such a thing is used expect a bit of the Wild West.

Recent answers Have been amazing.

The RCA connector - isn't it designed to accommodate one conductor and then gripping the shield where it is soldered at the base of the connector.  So how does the RCA connector accommodate 2 connectors(positive and negative) and a ground wire.

I really like atmaspere’s answer including discussion about the shield carrying the signal. It made a lot of sense to my simple mind.

@jumia   At one RCA connector shield and negative/return wire are both soldered at the base of the connector while signal/positive wire is soldered to center pin.  On the other side only two wires are soldered while shield is left unconnected/floating.

Since you are only concerned with single ended RCA interconnects with coaxial cables, I will share my experience with you.  

For many years I used a fine 75-ohm Belden #9259 oxygen free single center conductor coaxial cable with a bare copper braided shield over a polyethylene foam dielectric.  This was my principal preamplifier output to my mono block power amps input for many years.  I really had no complaints.  Due to a change in my room arrangement, I was forced to construct new & longer interconnects.  I was unable to acquire an adequate amount of the Belden #9259 cable without purchasing an entire roll.

I did however receive a timely tip from a respected analog guru.  He recommended Mogami W2549 coaxial cable which has 2 OF center conductors + bare copper braid.  You can experiment with it in 2 methods: 1) solder 1 center conductor to the RCA + cup, the other center conductor to ground & the bare copper braid as the shield or 2) solder both center conductors to the RCA + cup & the bare copper braid as ground.  Interestingly, the company that I purchased the cable from (Redco) actually terminates them per method 1 (above).  However, I chose the second method.

I made my main pre to power amp interconnects 25' long.  2 more pair 1 meter long and a 4th pair 2 meters.  After about 300-400 hours of "run-in" I was more than pleased.  Doing this project requires some good tools, materials and diligence or alternatively, pay someone or business to do it.  If you are a DIY person, you may wish to select an RCA connector with a crimp connector option for the braid so that you only have to solder the center conductor(s).  I found that soldering the braid to the ground of the RCA connector to be the most difficult part of the process, mostly because the RCA's that I chose didn't leave me much real estate to work in.

I was not able to compare these particular Belden vs Mogami coaxial cables in my system fairly, but I am very pleased and settled with this decision & execution. The Mogami cable is exceptional in every aspect with vivid detail and dynamics.  

I hope this has provided some limited comparative insight.

Best luck