I've spent years studying credit card rewards, and I'll be honest -- there's no one-size-fits-all answer to the cash back vs. points question. Some people swear by the flexibility of cold, hard cash.

You pull up your credit card app, see two offers: one gives you 5% cash back on groceries, the other offers 5× points redeemable for flights. Which do you pick? Depending on what you spend, how you ...