I think the decision of which book to get is best made on the bias of who you are likely to play with. If your group prefers D&D or simpler, destiny. Pathfinder up, ATOW. In-between is a judgement call
I'd just run, Mechwarrior: Destiny.
I'd just run, Mechwarrior: A Time of Destiny.
How well does Destiny do with Alpha Strike and TW.