From a technical standpoint, At least in my opinion shorting plugs would be better for the fact that they will ground all wires and traces leading up to the selector relays or switches on a preamp which is reducing possible interference from those areas. For obvious reasons you woudlnt want to use them on outputs. They are good to use on unused inputs on preamps just to be sure that you have the quietest circuitry possible. Most preamps come with them for use on tape inputs standard. Although if you dont want to spend big bucks for fancy ones you can get a bunch of those cheap rca cables that come free with most equipment. Lop the ends off and leave maybe half an inch of wire sticking out of the plug then solder the wires together.
Im not sure what advantage the cardas type without the center pin has. This is taken from their website...
Cardas RCA Caps fit over unused female RCA connectors in your system to eliminate EMI and RF noise absorption. Unlike traditional shorting plugs, these caps do not short the inputs (many pre amps do not like having their inputs shorted). Another difference is Cardas RCA Caps completely cover the RCA input with a solid shield of nickel and brass to prevent both EMI and RFI from entering the chassis through the RCA dielectric. This is a more elegant and probably more effective solution than either shorting plugs or wrapping the plug in aluminum foil. Our caps also protect the RCA from dust and corrosion.
I dont see why they claim that "many preamps dont like having their inputs shorted" Since most high quality preamps use relays it wouldnt make a bit of difference Unless of course the input was switched to the one that is shorted. That could possibly present a problem.
