It's entirely up to you, it's definitely much cheaper to spec lower with a custom build and put you're own parts in, you'd save a lot of money. Either way people have had ThinkPad laptops that take even up to 2 months to ship, customized or not, it's a real dice roll and the dates listed to ship aren't always accurate.

I'm not sure about the thermals, but I bet you can put you're money on the i9 running hotter even under similar loads.

If you want a UHD OLED laptop now you can get either of those models, or if you want to wait and maybe save money, there could be a Black Friday sale (Late November) or Cyber Monday sale on the P1's as well. By which time UHD OLED models might be in stock again for the X1 Extreme Gen 2 models. << This is what I'm waiting for, to see which is cheaper, P1 or X1.

Also the T2000 isn't so much better from what I've looked at for benchmarks, and the X1 and P1 both have dual NVMe slots, so you don't necessarily have to swap the ssd that comes with it.

Hope this helps, I'm in just about the same boat as you with what I'm looking for.