No resin content does not make a better result for surface finish. I think it is more down to the resin chemistry and how it is processed. 

Yes you can use surface for backing layers as I generally use surface only for my work. The down side is cost, 245gr surface will need more layers to get the thickness you need or strength. But then more layers of the lighter weight material can be oriented to give strength in different directions as you can align the weave in more directions for the same weight overall. 

The best surface materials just happen to be single sided...... Or that is the only real way to make it work.... Who knows? lol...