I´ve had both.
The X-T2 has a larger body with better grip and a place for thumb and is (much) easier to hold.
With the X-T20 you have touch-AF and touch-to-shoot, and it´s smaller, which all can be of use in street shooting.
I have the X-T2 now and am still not sure, if it was the right decision to sell the X-T20. It was better for street, the T2 is better for everything else, especially with larger lenses. You can buy an additional grip for the T20, but in my experience it doesn´t help much (still no place for the right thumb), it just makes the T20 a lot heavier. I think the ergonomics of the T20 are unfortunately screwed up a bit compared to Canon M5 or Sony A6xxx.
I´m planning to use the T2 with Canon glass as soon as my adapter arrives, so I guess I´ll keep it because of the ergonomics.