I also have a glass hydrometer, it is consistent at least and pretty accurate.
I also have an American Marine Pinpoint conductivity meter. Sometimes the Deep six swing arm with agree, sometimes it doesn't. I have found that bubbles tend to get on the arm and really mess up the accuracy.
A couple of glass hydrometers should be fine. If you want a gadget, get a refractometer or a conductivity meter.
That's a good point that the Donger makes... if using the Deep Six make absolutely sure that there are no bubbles on the arm. You can easily check this by looking from above and the sides.
