What PC should I choose?

[weabole]

https://www.amazon.com/HP-Performance-Pavilion-580-137c-Desktop/dp/B077XSH2ZQ/ref=sr_1_6?ie=UTF8&qid=1537130467&sr=8-6&keywords=rx+580+desktop
This PC carries 16 GB Ram, RX 550, Ryzen 7 1700.

Or

https://www.amazon.com/HP-Pavilion-Desktop-Computer-690-0020/dp/B07BHXV3TJ/ref=sr_1_5?ie=UTF8&qid=1537130890&sr=8-5&keywords=rx+580+desktop
This PC carries an RX 580, though 8 GB ram, and a Ryzen 5 2400G.

What PC should I choose for the best performance in games like Fortnite?

 

[volkgren]

Definitely the RX 580 would perform better even with the 2400G. The Ryzen 7 1700 won't give you the performance that the RX 580 will.

Both CPUs are enough to support both GPUs, so the CPU will make little to no difference in performance. The GPU will be the difference in performance and the RX 580 is over twice as fast as the RX 550.