The 5X and 6P has a huge difference of display. 5X has 5.2 inches screen with 1920*1080 resolution at 423pp, while the other one is developed for special phablet with 5.7 inches of 2560*1440 resolution of 518 ppi. The other difference is gorilla glass, 5X has older gorilla glass 3 while the other one 6P has gorilla glass 4.
5.2 inches, FHD (1920 x 1080) LCD at 423 ppi
5.7 inches; WQHD (2560 x 1440) AMOLED display at 518 ppi