I've never actually gotten a $50 coupon from Citi as a retention bonus. But I did get a $50 coupon w/ my last Citi AA MC which was an AA.com discount code. AFAIK, no fare bucket restrictions but a $250 minimum. Nice and easy to redeem.

My preference would be for the $50 coupon, paying the annual fee, canceling the card, and then getting a pro-rated refund of the fee. Can't do that if you just get a fee waiver.