If you choose a regular travel rewards credit card you know exactly how much it is in dollars. For example, the Barclaycard Arrival Plus® World Elite Mastercard® has a 70,000 bonus miles offer — that can be redeemed for any airline ticket. The miles rewards you earn with this card can be redeemed for airfare on any airline, anytime with no blackout dates, or for gift cards, or even for cash.
If you have the Virgin America co-branded credit card with a 10k 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. In general 10k is hardly enough for a ticket, so you’ll have to pay more to get it.
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 30,000 Miles, you get 60,000 Miles. So you can get even better rewards that that fixed sum you are usually offered.