With $500/month that means $6,000 per year. At a 1-1 earn rate (which is just about the best you can hope for at the moment) you will get 6,000 pts a year. At this rate of earning you will be probably be only interested in using points on economy fares, and given the $$ of surcharges that now apply to award tickets, the best you can hope for is a redemption rate of 2c per point. So you will get $120 worth of points a year.
Therefore, if you are going to have a credit card anyway - I'd say make sure that the amount you'd pay for a card with rewards attached to it is no more than $120 dearer than one without - that shouldn't be a problem. As for which one to get.... I'm afraid I'll have to defer to others.
From just $6 we'll remove all advertisements so that you can enjoy a cleaner and uninterupted viewing experience.And you'll be supporting us so that we can continue to provide this valuable resource :)