Bitstamp is an individual exchange. It is sometimes referred to as Bearstamp because its prices are often lower than other exchanges. The prices at different exchanges vary greatly due to location, features and other considerations.

Coinmarketcap is a price indexing service like Bitcoinaverage, Preev, or the Coindesk BPI. These indexing services use averages from multiple exchanges, weighted by volumes.

I only use  Coinmarketcap for altcoins because I find that Bitcoinaverage gives a truer indication of global prices, as it averages more exchanges and excludes volumes from zero-fee exchanges. Bitcoinaverage is also the base price used by the majority of BTC ATMs here in Toronto, so to me it's most representative of real-world prices.