Both...all of it...spend money on cyber monday...which do you prefer?Its up to you. Do you want to complete act 5 more than spend for deals and specials?
You don't need too much units for act 5 completion, 500 units will help you, specially for final boss og ultron
Imo and I am sure this is unpopular but isn’t it more exciting to get rewards from defeating content than stock piling units for an offer? Personally I say save your units for content and enjoy the game. Earn your position and rank in the game and forgo the short cuts of deals and offers. Maybe it’s just me, but I have always felt this way. My 2 cents
You don't need too much units for act 5 completion, 500 units will help you, specially for final boss og ultron Will 4 star maxed out red mags do the job?