There probably won't be much between them in terms of effiency since they both work at over 90% effiency at 50% load. It'll come down to your use case and idle:load ratio.
As you have already discovered, you can either desolder the fan cable of the 7660b and resolder appropriate cable or snip it and wire up a 2 pin connector.
I would do this anyway otherwise you're sitting on a fairly useless PSU in it's current form.
However :
The Silverstone FX-500 uses a 40x40x15mm fan and I'm guessing the ADDA fan from your ,presumably, 7140b is 20mm and so will not fit.
You could go ahead and find a decent 15mm fan such as one of these ones from ADDA, they even have a 15mm version of the AD0412HB-D51 which may be the best option out of the ADDA fans at least.
If you decide to mod the FX-500 then there's this teardown and fan replacement guide on reddit that you might find useful. Looks like there's a handy 2 pin slot so no soldering required. They use the Noctua 10mm in the guide and leave a gap which is a pretty bad move so don't do that.
WARNING:
I'm sure you're aware, but if you go sticking your fingers or screwdrivers in about the capacitors in those PSUs and you make an electrical connection then it's lights out/game over/death.
I'm not saying don't do it, just be careful.
