@Dingoblue 

Hello, simply comparing the Wi-Fi coverage, the RE650 may have a larger coverage than the RE505X as you mentioned also the RE650 equips with 4 external antennas.

But for your 2 levels larger house, we actually don't suggest you purchase the range extenders which may help little to extend the wireless coverage from the first floor to the second and we probably get a better experience when the extender works at the same level horizontally.

If you do need to extend the signal to different floors, the powerline extender kits or Deco mesh system would be a better choice:

https://www.tp-link.com/home-networking/powerline/

https://www.tp-link.com/home-networking/deco/