HV20/30 versus GL2
The GL2 has many more options available for shooting then does the HD HV20/30. I have only shot with the hard drive based Canon's in that range, but the hard drive based would only shoot in HD. If you can shoot the HV20/30 in SD then that could be an option. HD means more storage due to file size.
I can say that the quality of the one I shot with HV10 (I think) was excellent. However, there is no manual setting for the audio inputs on those. If you are going to do an audio you will find that you need XLR inputs, and while the GL2 doesn't have them built in, there is a handy product that fits right in the hot shoe that works. I find that we need to adjust the input levels on the camera as well as the field mixer. It is also easy to switch to manual focus, set the white balance, etc. many functions that are tough or impossible to do on the HV 20/30.