On average I value an airline mile at $0.0225, but the valuation depends on how the miles are redeemed and some options (that I would consider) are as low as $0.0175. Nonetheless, I prefer miles over cash or near-cash (e.g. Capital One Venture) because:

1. Comparing a cash back credit card to a mileage card isn't merely a matter of calculating the value of "a" mile, you need to look at the full package. Cash back cards seldom offer promotions, but mileage cards often have bonus offers for spending in targeted categories, for achieving annual thresholds, or for transfers to specific partners.

2. The premium class ticket that you buy with cash is very likely to require an advance purchase, be non-refundable, stipulate a large fee for changes, and cover only point-to-point travel. By comparison, mileage tickets often (not always) offer much more liberal terms and only trivial penalties for cancellation.