They're both so beautiful in their own way.
Mega evolutions pops and feels very Alice in wonderland on an acid trip, and 151 feels very natural and cute, like you ran into bulbasaur/ivysaur/venusaur in the wild in their natural element.
I think 151 venusaur is my favorite out of all of them but overall I prefer mega evolutions because the art is unique and I like the color scheme more.
Hbu?
by Sgt_Bulbasaur