And if you use your credit cards heavily to take advantage of reward offers, you could be penalized, even if you pay the amount in full every month.

WSJ: Getting Going: Why the Latest Credit-Score Fixes Fall Short

For the purchases that you will not be able to pay for in full each month (large purchases that amount to loans), go with a personal credit card because of the predictability of its interest rates on existing balances.

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You can work out default expectations by looking at the spreads on speculative bonds and making an assumption about the amount investors will recover from companies that fail to pay up in full.

ECONOMIST: Buttonwood