The DX220 seems to be a stopgap device just for clearing out the old processor and DAC inventory. DX220 used back the same RK3368
8-core processor as the DX200, while the N6ii used a
4-core Snapdragon 425 but the 4 core N6ii is still faster. Why? The DX220 is severely underclocked and yet gives out more heat.
The smoothness when navigating and interaction with third-party music apps is apparent on the N6ii.
I assume people who are interested in both DAPs are going to use some apps so unless you can live with the lagginess, hotness, mediocre battery life, then I wouldn't touch the DX220.
Sound comparison is up for debate and choosing one is a matter of iems pairing and personal preferences. And I have seen quite a number of people switched from DX220 to N6ii and not the other way round.