Exit row has a bit more legroom, but for me, premium offers plenty. I do like the exit rows (as well as the free drinks in premium), but agree the snack isn't anything particularly amazing or worth selecting premium for over the exit row. I like being closer to the front so will generally take premium, unless I can't get an aisle, then I'll stick with exit row if there's one there. Empty middle definitely depends more on the route/flight load, but I've seen an empty middle in both exit/premium seats on occasion.