I asked FXCM for an explanation of the datafeed for their charts and this is what they said:
The S&P Comstock feed is an indicative feed that takes a composite of bank prices and streams them in real time. The Interbank market is not exchange-based - banks and large market maker such as FXCM trade with these banks on an individual basis using a Reuters or EBS platform. Each bank shows their own bid and offer, therefore you will not see a centralized price. If you are looking for a powerful, stable charting package with a composite feed from over 200 different banks and market makers, you would find it in the eSignal Premier charts.
Is it better to trade based on a chart that displays the dealers bid price, or would it be a better and more accurate way of trading to trade using one of these "composite feeds"?
The problem with retail forex brokers is that they control the prices (the bid and ask) that they give their customers and show on their charts. Because most if not all of them manipulate the price, their prices can get slightly out of line with what the interbank market is trading.
The prices that you really want to look at no matter who you're trading with (retail FX, futures, interbank) are from Electronic Broker Service (EBS). EBS is the spot platform for banks that does 1/3 of the daily spot volume at about 130 billion per day. Bloomberg pro has it, Reuters may have it, and I've heard that CQG will have it soon. There may be a cheaper chart pakage that has it, I'd check E-signal for starters. For free real time sites, I think all you are going to find is stuff that originates from retail fx brokers; better than nothing but definitely sub par.