Obviously they are the same phone with a few exceptions
Mate 20X vs Mate 20 Pro:
7.2" vs 6.4" display
174.6 x 85.4 x 8.2 mm vs 157.8 x 72.3 x 8.6 mm
5000mAh vs 4200mAh
Fingerprint rear vs on screen sensor (unfortunate but understandable)
3.5mm vs USB-C (nice!)
IP53 vs IP68 (that's disappointing but understandable for the proposed price cut)
1080 x 2244 vs 1440 x 3120 (that actually suits me, lower res = better battery generally and possibly helps cut the price)
Water drop notch vs no notch
18.7:9 vs 19.5:9 ratio
Both have the supercharge and identical cameras, cpu, GPU etc.
Early reviews suggest the 20X will be cheaper but I am not convinced. It misses some features but it's larger and will be newer. Do we really expect it to come in cheaper?
Me personally I'm thinking I could get used to the larger size. Battery life will be awesome, possibly even better than the 20 Pro. Reviewers have been impressed.
The difference will sort of be like a Samsung Galaxy vs. Note phone, really.
Whether the 20X is cheaper the 20 Pro, nobody will know. I think it's likely because the 20 Pro is the flagship phone, whereas the 20X seems to be a "big" variant. Bigger phones are easier to make from an engineering standpoint, screen resolution is lower, it drops some of the marquee features like the fancy fingerprint scanner and the high-res screen.