Backpay....
I suggest y'all do your research on the history of back pay.
While "full back pay" isn't in the "never happened " category, it is not the norm. In fact, I've personally seen it be pennies on the dollar.
If financial gain is your priority (as asking for backpay would suggest) one is far better served by "contract now" vs. retro pay.
Plus, when you factor in improved 401k, travel pay and per diem, it's not even close.