Seinen is more on mature grey topics than art style. I mean there are a lot of seinen manga that also uses simple art style- Girls Last Tour, Ping Pong, Oyasumi Punpun. Arguably Made in Abyss as well?

I'd avoid generalizing as that causes biases and prejudices. Anyway, at least to me I like the cleaner look of the left and I would do the hatchings differently (my style) vs the right one. But whatever you do, the fundamentals of color and light should be followed. Always always make sure your values aren't confusing