Ok, here are the results from my testing. Each test was conducted in a small sampler glass. Some beer pulled and discarded before every pour so that the lines were cold. Temperature of keezer was at 37 degrees F.
Perlick 525SS, 12ft 3/16 Tubing, 10psi
Perlick 650SS, 12ft 3/16 Tubing, 10psi, flow control slowed to half speed
Perlick 650SS, 3ft 3/16 "Silver" Tubing, 10psi, flow control slowed to half speed
Perlick 650SS, 12ft 3/16 Tubing, 10psi, flow control on full speed
Perlick 650SS, 3ft 3/16 "Silver" Tubing, 10psi, flow control on full speed
Perlick 650SS, 12ft 3/16 Tubing, 30psi Beer Gas (Nitro), flow control on med speed
Stout Tap, 12ft 3/16 Tubing, 30psi Beer Gas (Nitro)
Nitro Stout tap (Left) vs Nitro 650SS (Right)
I had a problem with my nitro tap while doing this test, I was getting pretty much all foam. I was getting a little bit of the cascade effect, but not as much as usual. I'm pretty sure the (subpar) test beer I was using was just carbonated a bit too high for the Nitro stout tap.
Either way I think it's clear that the nitro tap gives you a creamier head than the 650SS on beergas, although I did see a tiny bit of cascading with the 650SS sample. I will definitely be keeping my nitro tap, I think it gives better results.
I'm pretty pleased with my new 650SS taps though. I can get a good pour with only 3 feet of tubing, so that will tidy up my keezer immensely. I think the 525SS, when psi temperature and tube lengths are all perfect, produces a slightly better and longer persisting head than the 650SS. Also I think my 650SS taps were dripping slightly more than the 525SS. Overall though I think the pros of the flow control are definitely worth these minor issues.