Hi, and welcome. I have two things for you to consider:
1) My library does offer ebook rentals. However, my Sony couldn't read any of them, because they had some sort of DRM protection that Sony couldn't read. So, if being able to read library ebooks is important to you, make sure the device you order can read through the kind of protection your library puts on the books.
2) The big difference between the Sony (that I have) and the Kindle (that I want) in my mind, is the ease of getting books onto it. You might not think it would be a big deal, and maybe for you it wouldn't be, but having to sit down at my PC to buy a book, and then have to connect a cable and do a transfer is just a huge pain. Frankly, as a result, I don't use my Sony Reader as much as I might otherwise. The thought of being able to get books instantly and wirelessly is a HUGE plus for me on the Kindle side. So consider what is important to you in this respect.
Either way you go, I bet you will love your ebook.