AFAIK vendor lock-in is only a problem in the Amazon/Kindle ecosystem.
Buy a reader that supports EPUB and PDF and only purchase books that come with an EPUB download and you won't have any DRM or lock-in to worry about.
The Kobo store is a pretty good place, I also pick up a lot of old books from Project Gutenberg. I use Calibre to manage my books.
I've tried both a Sony and a Kobo ereader. My Sony wins hands down for web browsing and in general refreshes better. The Kobo I tried had a nicer home screen and some games like chess and sudoku which I thought was cool.
I rather doubt that the Glo is significantly slower than a more modern reader so I'd stick with it.