Blackjack Odds & Such

At 888 poker if you deposit on Wednesday you get 20% cash back up to $20 if you lose (no wager
requirements) the odds are clearly in your favor if you play smart, blackjack has a 0.5% house advantage at this casino, would it be best to

A: bet the whole $100 and not be able to double down

or

B: bet $50 so you are able to double down

Also if the answer is B if you lose I"m guessing you would then bet $25?
And at what point is it no longer +ev to keep playing?

Thanks!