I have a 120 and 220.
I've compared the 120 with other coarse stones like the Nubatama 150, Imanishi 220, Atoma 140, and DMT 120. IMO, all of these stones have very similar cutting speed with the Atoma being the most powerful and followed closely by the Shapton.
The 120 and 220 glass are very hard stones that wear very slow, probably the best dish resistance of any coarse stone I have used. (Dish resistance is the stones ability to resist wear from sharpening allowing it to stay flatter longer). They have a wired feel though, almost slippery and it will sometimes feel like the knife is skating over the stone. It's deceiving because when you look at the bevel there is a bunch of steel missing
If you have a 500 then the 220 is the logical choice. If you feel the need for the 120 then you would likely need the 320 too before the 500. 120-500 is often too big of a jump and edge quality suffers along with making you frustrated.