For myself, I stick with firmware 2.4.0 for several reasons. I was pretty happy with 2.5.1 but I also use Calibre, and Calibre always gives an error that it "cannot communicate with device" for WinXP users who also have AMD cpus. It was fine on my Windows 7 netbook, but I rarely use the netbook so it got to be quite a hassle to get it out every time I wanted to transfer books to the Glo. The other thing for me is that I prefer swipes to turn pages, and with 2.5.1, I'd have to swipe up to 4x to turn a page, and that wasn't a rare occurrence. At least 50% of the time I would need more than 1 swipe. With 2.4.0 Calibre works perfectly for me, no errors communicating with the Glo no matter how many times I send or delete books from it. And while reading I noticed that it rarely required more than 1 swipe, so for me, 2.4.0 is ideal.
Other differences after 2.5.1 is that they changed the homescreen to tiles, which I think are just hideous. The homescreen before consisted of 5 books, and I use it as a queue of my next books I want to read. Clean and simple.
Kobo changes firmware ridiculously often, and instead of fixing past bugs, the new firmware usually introduces new and bigger bugs. Right now it's recommend that you don't install the newest firmware if you download .kepubs from Kobo. Some firmware versions had bigger gaps at the top and bottom of book pages, some had light issues (not being able to be low enough for some to read in the dark), font issues, etc. I normally keep my light at about 28% and it suits me for all reading situations. The Glo updated to 2.10 when I had to do a factory reset and to get the same amount of light, I had to turn it up to 71% just as an example.
But what bothers me might be perfectly fine to you, you might prefer the tiles on the homescreen. I can only suggest you hunt around for screenshots of the home pages and then just try the firmware and see how it performs for you. And check the
Kobo Forum here for more information. There's not one place that lists all the pros and cons of each firmware version. But when one does come out, there's usually a thread about it and if there's any problems, they're bound to be posts about the bugs. There is a
thread with links to older firmware in the Kobo forums with instructions how to go back to an older version. It means hardly ever turning on the wi-fi for my Glo, but I prefer to purchase all my books through my PC and sort them out in Calibre and then sideload them always to my readers, so that's not an issue for me at all.