Credit Rating versus Credit Spread

A Credit Rating is typically a more statistically driven measure of Credit Risk, used within a broader Credit Rating System and typically be defined on a limited Rating Scale
A Credit Spread is a market based metric that either directly (in the case of derivative instruments such as credit default swaps) or indirectly (in the case of credit sensitive bond / loan products) derives a market implied degree of credit risk

Pros and Cons

Market driven measures tend to respond more rapidly to new information
Market driven measures may have excess volatility versus the actual credit risk
Markets may be subject to various pathologies such as bubbles, cornered markets etc. which may diminish their risk sensitivity