Slip Streams are supposedly softer in noise signature, and I can vouch for the 800 RPM SS I have vs the S-FLEX Es I ran at similar RPM for gaming. The SS makes more of a whoosh sound and the S-FLEXs mix in a bit of mechanical noise. Much better at sucking air through filters or when mounted horizontally though, less RPM drop vs my (admittedly weak) SS.
Both are still smooth as silk to my ears, it's like comparing different steel alloys for hardness to me. Today I would pick the PWM option
though, no contest.
Neither of my fans has ever had clicking, even though the S-FLEXs have gotten really dusty at times. Could be sample variance as suggested.
Since the human ear adjusts to loudness if you let it, the signature is what counts (how annoying the sound is). Pick what you like best - I'm currently scooting along fine on Arctic Cooling F12 PWM fans that are not as silky-smooth, but do the job just fine for half the money.