I decided to buy myself an Aura, and make my sister happy with my Glo. Here are the differences I have noticed:
- Like people have mentioned, the light on the Aura can go dimmer
- The light on the Aura is also a different "color", the light on the Glo is kind of harsh/cold (blueish?) and the light on the Aura is softer, even when you make it brighter. The Aura's light is much nicer on the eyes in my opinion
- The Aura does the big loading flash less often (you can set it for 5 pages, 10 pages or every chapter)
- The Aura is slightly smaller, thinner and it's lighter (same screen size though)
- The Auras screen is level with the sides, whereas the Glo's screen is sunken
- Different touch screen technology, the Aura reacts only to your fingers (and stylusses I suppose), the Glo reacts to any touch. Personally I prefer the Aura here, because now I won't suddenly end up pages ahead if I accidentally put my Kobo face down on my bed

- The Aura's touch screen also seems more accurate, easier to highlight the right section, etc.
I think that's it.
Conclusion:
I loved my Glo, and if it wasn't for my frequent migraines with extreme light sensitivity, I probably would have kept it and happily used it for years to come (especially with the night mode installed). But I do have those migraines, and the Aura is just that bit nicer on my eyes that for me upgrading to the Aura was worth it.
If you don't have a Kobo and you need to decide between a Glo and an Aura, I would definitely recommend the Aura.
If you have a Glo and the light is slightly uncomfortable for you, I would say upgrading to the Aura will be worth it.
If you have a Glo and have no issues with the light, I'd say: Just keep the Glo.