Tom Hanks is a more laid back type. It might be his comedy background showing through.
Gibson has the intensity needed to portray a good battle epic.
I would suggest using unknown actors like they did in Band of Brothers, that way they could be directed by him to give some unforgettable scenes.
Use good acting and direction, aided by select CGI, and a great movie could be made fairly cheaply.
The battle scene in We Were Soldiers was intense and so damn moving.
https://www.youtube.com/watch?v=LzKr_uP6lAg