The Clara HD is definitely an improvement over the Glo HD. But, I don't think it is enough of an improvement to justify replacing a perfectly working device. If one of the extra function is something you need, then it could justify the upgrade. I have both, and while I didn't upgrade directly from the Glo HD to the Clara HD, it is better.
Having said that, and rereading your post, the answer is obvious. So, here is a version you can show whoever manages your finances :
The Clara HD is a much better device. The screen is much better and has very little ghosting. The Comfortlight Pro light means you will sleep better. The Overdrive integration means you can borrow books from the library rather than buy them. The extra space means that you can carry more books including those big books you haven't been able to read. And it will mean you can stop swearing each time you need to put a few books on the device and the USB port doesn't connect.
Will that work?
And in case you are wondering, I'm very familiar with the question, "Why do you need another ereader?"