British Airways rewards program uses a rewards currency called Avios. If you have the British Airways co-branded credit card with a 50k bonus offer, it does not guarantee that you will get a round-trip award ticket. The reason is that the redemption options and required number of miles may vary depending on destination you choose (domestic or international), flight class you prefer (economy or business) and the ticket price.
If you choose a regular travel rewards credit card you know exactly how much it is in dollars.
Also you can pay your attention to travel cards with some unusual bonus offers, like Discover it® Miles. The card does not have a traditional sign-up bonus but all miles you earn your first year will be matched at the end of the year (provided you are a new member). For example, if you earn 35,000 miles, you get 70,000 miles. So you can get even better rewards than fixed sum you are usually offered.