One is an iPhone 11 replacement, and one is an 11 Pro replacement. Just they both have the same OLED screen now, instead of the LCD-OLED split. I think most people prefer OLED thanks to the deep blacks. I don't like the look of LCD screens on phones now due to the obvious backlighting, and wouldn't go back.
As for cost, presumably the cost/supply of OLED has come down enough now that it makes more sense to just make one common part.
If I have to go to a large phone to get "all" of the features, it's going to be a while before I give up the 11 Pro that I intend to buy.
I've had the X and XS and have been completely happy having all the features of the Max phones, while being able to use it one handed and fit it in my pocket.