The associations between the brokers and IBES / First Call are voluntary - brokers are not required to submit their analysts' forecasts to these organizations. So, the populations of broker/analysts differ between the two databases. Of course, the variables covered and the time series differ as well. (For example, the First call contains company guidelines but IBES does not, but IBES contains more forecast variables and a longer time series.)
Keep in mind that Thomson Reuters reports that effective December 1, 2008, First Call historical data will no longer be updated or revised. New monthly historical cuts will continue to be appended. Over time, Thomson-Reuters plans to phase out First Call database and focus on maintaining IBES database.