The GPS chip used in the Android devices (usually SiRF Star 3/4) are of civilian grade. Civilian grade chips has some deliberate limitations in par with military grade chips. The civilian chips does have some intentional errors called "Selective Availablity".
Hence, whether it is Android or iPhone or Bluetooth GPS receiver or a dedicated unit which could be handheld or the one in car comes under civilian category which cannot be more accurate than 20 meters under the best possible satellite lock (of course, with AGPS it can be improved). Please check the Wikipedia article which says "...improving the precision of civilian GPS from 100 meters (330 ft) to 20 meters (66 ft)."
iPhone uses BroadCOM's BCM4750 chip and Android phones (Samsung Galaxy S2) uses SiRF Star 4.
Interestingly, a web page claims "The published steady state position accuracy of the BCM4750 is 2m.", which in my opinion could not be correct. In this research publication it is said as "iPhone 4 exhibits an offset (of about 20 metres)", which seems to be more scientific and correlates with the Wikipedia page.
However, all these talks are about hardware aspect only. It is the software that reads the input from chip, processes it and displays. If it is not doing a good job, well, still it spoils the show.
So, finally my answer would be again be a question "If all these civilian GPS devices cannot be accurate by more than 20 meters, does it mean anything to say X is good and Y is bad? ;-) "
Update Thanks to @Ropo I observe that Selective Availability is indeed removed. Here is the official statement, which says:
Selective Availability (SA) was an intentional degradation of public GPS signals implemented for national security reasons.
In May 2000, at the direction of President Bill Clinton, the U.S
government discontinued its use of Selective Availability in order to
make GPS more responsive to civil and commercial users worldwide.
The United States has no intent to ever use Selective Availability
again. In September 2007, the U.S. government announced its decision
to procure the future generation of GPS satellites, known as GPS III,
without the SA feature. Doing this will make the policy decision of
2000 permanent and eliminate a source of uncertainty in GPS
performance that had been of concern to civil GPS users worldwide.
Please take a look at a question at StackOverflow as well.