More durable shaft - difficult to accidentally break off

Must use knobs with set-screws

Most popular type of replacement pot for Fender amps

Used in all vintage Fender blackface amps, and most vintage silverface amps

Strongly recommended for use on amps which are used by musicians who play gigs and concerts, and therefore subject the amp to possible bumps and dings in travel or on stage.    It doesn't take much of a blow to break off a nylon pot shaft.

If your amp is used only at home, the nylon-shaft pots would be fine.    But for added protection and peace-of-mind, the metal-shaft pots would be the choice.

No difference in sound quality or feel when compared with nylon-shaft CTS pots.

All things considered, the best pots for the job.

Less durable shaft - easier to break off

Fine for use on Fender amps which are used only at home and not subjected to the possible bumps from travel, set-up on stage, etc.

Can accept set-screw or "knurled" type knobs

On average, about 25% less expensive than metal shaft pots

Found only in certain models and years of Fender silverface amps.  Fender used them for a period of time, then went back to metal shaft pots

Not found in Fender blackface amps (1963-1967) or early silverface amps (1967-1968)

It is not possible to date an amp by whether or not it has nylon shaft pots.   Fender used them intermittently from the early 70s until the late 70s.

The set-screws in set-screw knobs make a "dent" in the side of the nylon shaft.   No harm done by that dent.   Push-on Fender knobs numbered 1-10 (commonly called "witch's hat" knobs because they resemble a witch's hat) are pretty rare these days.   They tended to pull off of the pot shafts too easily, and so Fender did not use them for very long.

Especially after push-pull master volume controls were introduced around 1972, the push-on knobs could not be used, because the knob would easily pull off the shaft when trying to operate the push-pull feature.   So, Fender went back to the set-screw knobs

Fender nylon-shaft knobs from the 1970s had different color shafts depending on the pot.    Red, blue and green were common.   The colors, however, didn't mean anything.  Today's replacement nylon shaft knobs all have white shafts.

Fender, under CBS ownership at the time, was attempting to save money by using less expensive nylon-shaft pots, and less expensive push-on knobs without set screws.   The knobs were discontinued first, due to the ease of accidently pulling them off as noted above.  

Nylon shaft pots were discontinued because of customer complaints about shafts easily breaking off if the amp were accidentally bumped at the knob level.

For people wishing to keep an amp with nylon shaft pots as true to original as possible, the CTS nylon shaft pots serve a purpose.

No difference in sound quality, feel, or long term life (other than shaft breakage) when compared with metal-shaft CTS pots.