I've got a set of NGK APR6FS plugs in my Pinto at the minute, which work well and are what the car was rolling road tuned with.

As part of a full service I bought a replacement set, but managed to pick up AP6FS (no R).

Anyone know what the difference is and whether I'm likely to see any difference with them fitted...?

IIRC
R = Resister built into the plug which stops interfierence

If your running a distributor with points then you would be OK with non resistor plugs. If you are running an engine with an ECU you will need the resistor plugs. Some electronic distributors may?? be OK with non resistor type IIRC the resistor is to cut down on electrical noise which can affect ECUs