The Banjo flow meter http://www.dultmeier.com/products/0.802.806/1361 is a magnetic style meter.  I have found it to be very accurate throughout a wide range of flow rates.  The issue mentioned in this thread http://talk.newagtalk.com/forums/thread-view.asp?tid=62168&mid=438486 is more than likely a result of the meter being switched to read liters instead of gallons - at least that was the case when we experienced the same scenario.  For best accuracy, when installed the face of the meter needs to be angled towards you (like 45 degress or something) as opposed to facing directly upwards.  12"-18" of straight hose before and and 4"-6" of straight after the meter and it will be quite accurate.  I have one [flow meter] mounted after the pump between a Batchboy and a venturi inductor, both of which drastically influence flow rates and I have found the meter to be very accurate 99% of the time.  I had one Banjo that got moisture in the display and they replaced it without batting an eye.

The Raven flowmeter that I had was an older one,  2" with pipe threads, and it was rated at 100 GPM. http://www.dultmeier.com/products/0.802.803/1360 is available as a full port flange rated at 5-200 GPM.  I believe the Raven is a turbine type meter, which is vulnerable to obstructions.  I may be wrong on this, as I have not see a new one.  The controls on the new one appear to be the same as the one I had.

I personally like the GPM feature that the Banjo has, which allows you to view GPM at all times, in addition to total batch gallons.  The Banjo uses AA batteries that you can find in the house, eliminating the trip to Radio Shack in search of a 3.6v Lithium battery.  I enjoy equipment that requires little memory or experience to use, which allows my 85 year old Grandpa, my wife, or other part time help to be of service without requiring constant refresher courses.  The Raven has three buttons, with no written labels - only hieroglyphics - that perform a myriad of functions which may or may not be needed.  The Raven comes with a 8 1/2" x 11" laminated instruction sheet with instructions on both sides http://www.ravenprecision.com/Manuals/pdf/016-0159/873b.pdf.  My dealer took both Banjos out of the box and handed them to me and I have never once had a need for an instruction sheet.

The Raven may perform the task as required, but in my experience, the Banjo is a better product for most people's needs.