Thanks for the links. In the second one, I actually see there is a hike in the CPU hertz *3.6 GHz to 4.0 GHz), on top of an upgrade to the GTX 1080.
In my opinion and experience, $1500 - $1600 USD (around 39 990 CZK) is a great deal for that PC, as is $1200 - $1300 USD (31 990 CZK) for the previously mentioned PC. It is up to you. If you want faster computing power (for $300 USD, 8 000 CZK more), then go for it. X-Plane 11 does not need it, but could certainly benefit, especially with X-Plane 11, which is already on track to be quite the performance hog so far. :-)