It depends. If you're a smaller individual like the FTer who started the "I prefer middle seats" thread, probably so because as you point out, the windowless wall is an even surface.

For those of us of the "Northern European" size category, it depends. The window gives an extra inch or two of shoulder room if it aligns correctly. The windowless seats also tend to have the bulge between sections, which further cuts into shoulder room.

My personal favorites are the rows with two windows, where the rearward window is roughly even with my shoulder with the seat upright-ish. I can lean into that window in a futile attempt to sleep, or I can lean forward and look out the other one.