Don’t cancel your Citibank card. At least not yet…
Just because you got an offer doesn’t mean that you will qualify for card. You got the offer as part of Discover’s pre-screening for potential customers.
It’s very common to get a tempting offer, but to end up with a lesser card. It’s o.k. to give Discover a shot, but don’t be surprised if you end up with a similar card to your current Citibank card.
By the way, it’s okay to have more than one credit card. Use both cards for small purchases such as groceries, gas, clothes and such. Wait for the statements and pay the balance in full each month. That builds credit and avoids interest.
If you choose to close a card, make sure go get a written confirmation that it was closed with 0 balance (See close-credit-card.html for detailed information on how to close credit card accounts).
Lastly, if you carry no balances on your cards (which you shouldn’t), then the cards’ APR makes no difference anyway.