There are a number of reasons to prefer satellite data over surface data for temperature measurement –satellites have better coverage and are not subject to site location biases. On the flip side, satellites only have limited history (back to 1979) so it is of limited utility for long-term analyses. Also,they do not strictly measure the surface, but the lower troposphere (though most climate models expect these to move in tandem). And since some of the technologies are newer, we don’t fully understand biases or errors that may be in the measurement system (though satellites are not any newer than certain surface temperature measurement devices that are suspected of biases). In particular, sattelites are subject to some orbital drift and changes in altitude and sensor function over time that must be corrected, perhaps imperfectly to date.
To this latter point, what one would want to see is an open dialog, with a closed loop between folks finding potential problems (like this one) and folks fixing or correcting the problems. In the case of both the UAH and RSS teams, both have been very responsive to outside criticism of their methodologies, and have improved them over time. This stands in stark contrast to the GISS and other surface temperature teams, who resist criticism intensely, put few resources into quality control (Hansen says a quarter man year at the GISS) and who refuse to credit outsiders even when changes are made under external presure.