Which rate is the best rate to go by for Bitcoin prices
Mtgox's price is high because it is difficult (if paying a 5% "emergency withdraw" fee) to impossible (if not paying a fee) to withdraw USD.  This makes USD on Mtgox less valuable than cash in hand, and therefore bitcoins are worth more in terms of "Mtgox USD" than real USD.

BTCChina's price has been high recently due to huge demand in China and capital controls that make it difficult for arbitrageurs to move USD or CNY out of the country.

When deciding on a fair price, it is best to use the price someone would get if they used the most convenient exchange in their country.  In the US, Coinbase or Bitstamp's rate makes sense.  In Europe, Bitstamp is the best option, and in China, a Chinese exchange would be more appropriate.