Let's start of by mentioning that the creator of Glo and Ilumina is one person, so you won't hurt his feelings if you choose one or the other. I was expecting Glo to be one step above Ilumina, and in a sense it is, but another sense it is not.
Glo is better than Ilumina in the sense that you can go to a chapter and verse in quick easy steps, and you can open multiple sessions of the Bible (and other lenses for that matter) at the same time. Also, Glo lets you select a passage in the Bible and drop it to all of the lenses to see if anything in the passage is connected to the atlas, media, etc.
What is lacking in Glo that is in Ilumina are the HD animations, guided tours, and the Bible Encyclopedia. Glo does have an encyclopedia based from Zondervan's Pictorial Encyclopedia, but it is only for some and not all articles whereas Ilumina has the entire Encyclopedia available.
In summary, for those who have Ilumina and not sure if Glo is worth it, for $35 Glo can be a perfect companion of Ilumina.