In my experience, I've found it best to go with the GPS fix unless it's not available. I don't know much about other location providers, but I know that for GPS there are a few tricks that can be used to give a bit of a ghetto precision measure. The altitude is often a sign, so you could check for ridiculous values. There is the accuracy measure on Android location fixes. Also if you can see the number of satellites used, this can also indicate the precision.
An interesting way of getting a better idea of the accuracy could be to ask for a set of fixes very rapidly, like ~1/sec for 10 seconds and then sleep for a minute or two. One talk I've been to has led to believe that some android devices will do this anyway. You would then weed out the outliers (I've heard Kalman filter mentioned here) and use some kind of centering strategy to get a single fix.
Obviously the depth you get to here depends on how hard your requirements are. If you have particularly strict requirement to get THE BEST location possible, I think you'll find that GPS and network location are as similar as apples and oranges. Also GPS can be wildly different from device to device.