Very little. Other than stated above. Think there is one less slider. Think the resolution of the screen is the same, but it is a bit smaller.
My buying advise:
Budget- Get the X18. Best price on an Ethos radio. This is a really nice radio.
If you have bluetooth hearing aids and would like to pair them to the radio, then get the X20S as it has this option.
That is my simple way of looking at them.
For me I might get the X18S as the extra memory might come into play as I have a large fleet of planes I tend to store in my radios, and most of them have complex mixing that takes up some extra space. I'm not the guy that typically has a flashy radio, my current radios are an old X7S and an X-lite S. Kind of funny considering I own a hobby shop but use under $200 radios.
I really like the Ethos radios, I don't think you will regret any of them. The build quality is great, they look great, and they are easy to use compared to all other radios on the market regardless of price point. I have always said you can compare a $100 FrSky to a $2,000 radio from some other brand and the FrSky will come out the winner. With Ethos, added in, we are now easy to use too. OpenTX was a challenge for many.