Since Lenovo only gives you the option of 1200p and the 3k OLED I'd personally go with the 3k display. Not because it's better, but because I dislike 1080/1200p more.

I am NOT a fan of OLED displays on laptops, but 1200p is not enough for me IMO. It's in an awkward spot where I non scaled text can be a bit small and I'd want scaling. But (in windows) the next scaling option is 125% which is too big. Plus the 1080p screen Lenovo gives you is typically hit or miss on quality. I'd hope the Z series are better than T series in that regard.

Since you're using linux the 1200p screen might be an advantage because text in text mode won't be microscopic without doubling it's size, and I'm not sure how all desktop environments handle scaling.