I have always used Novus, love the stuff. Works perfect. I'll have to look into this formula 21 stuff, sounds like a decent product for sure!
Novus is a 'Destructive' abrasive polish. Meaning, it contains fine grain grit/sand which removes surface-material when you use it. Its good for plexi control panels... and you may need it on beat up pins as well... however, once your pins is cleaned good,and polished to a smooth surface... then you want to try to stay away from anything that will further remove the playfield surface.
F21 is more of an Additive or Topical solution. It contains no gritty destructive materials. The only thing it will remove, is dirt/grime. It fills in the small pits/cracks and lays down a slick durable top coat surface for the ball to glide over.
Wax also a topcoat material. However, Wax isnt as durable, and can easily be pushed around, removed, and carried around... by the fast spinning ball. Its also thick and gooey, trapping dust, dirt, metal flakes, and carbon soot. This ends up traveling all over the place, as the ball carries it around. The harmful crud can end up getting pushed deep into the playfield surface. The pushed wax also tends to get pushed into and under things.. collecting on once pretty Posts, making them look horrendous. Because the ball gets coated in wax, it also carries the crud onto all the rubbers and parts that the ball contacts. Wax might make a not-so-smooth surface a bit faster... but, it wont last very long, as the wax gets moved and dirty. And the benefits deteriorate quickly, unless you remove all the wax, clean every part of the machine, and re-wax... every few weeks to a month, depending on play.
F21 is thin, doesnt trap material... leaving anything bad to get pushed under stuff at worst. Its so slick and durable, that the ball cant easily scrape into the field like a sawblade. Even if it could/does move via the ball, it cant trap particles within it, so it is not going to lift and smash crud all over the place. Its not going to flake or build up like wax. It can be cleaned and re-applied with ease, using another F21 wipe down.
F21 is more like a heavy-Duty, durable, slippery, lubricant. But it also makes for a very nice shine, and it even has UV light damage protection.
It can also be applied to your cabinet art, to keep it shiny and protected from UV.
I cant recommend it enough. One application, and you will never go back to wax.