There are two main considerations when figuring out how future-proof your new eReader is going to be: File Formats and DRM.
A file format is a way of describing how the text and formatting of an eBook has been stored in the file so that it can be decoded by the reader and displayed on the screen. There are many file formats for an eReader maker to choose from (plain text files, HTML files, PDF files, AZW files, Mobi files, and EPUB files are some major ones. Wikipedia has a more-or-less exhaustive list).
For the most part (especially when comparing Nooks and Kindles), file format doesn’t really matter. EPUBs can be easily converted to Mobis and Mobis can easily be converted to EPUBs with no-to-very-little loss of formatting using a program like Calibre. Both readers support reading PDFs though it’s not a great experience (the Kindle DX is much better for PDFs because of its larger format display; but for my money, nothing beats an iPad for PDFs). Calibre can also convert PDFs to EPUBs or Mobis, but there are often significant formatting issues so I can’t recommend it.
Many, many, many people talk about the file formats for Kindles and Nooks being incompatible; but I really don’t understand it. You can convert one to the other in a few seconds and then you’re on your way. EPUB or Mobi is just not a big deal.
There is a big deal, though, and that’s DRM. DRM, or Digital Rights Management, is a method that all the major eReaders use to attempt to prevent you from spreading an eBook around the Internet. Essentially, the file is encrypted (or locked) with some secret. Sometimes, the secret is a device ID. Sometimes, it’s the credit card number you used to buy the book. Sometimes it’s a number that the services ties to your account and keeps on the Internet. But this is where the real incompatibilities come in: until you decrypt the file, you can’t read it. Its file format doesn’t matter.
Amazon has created their own DRM scheme for Kindles that they haven’t licensed to anyone else. If you buy a book from Amazon, you can read it on a Kindle and nowhere else (because no one else will know how to decrypt it). In practice, this isn’t a big deal: Amazon has Kindle software for Windows PCs, Macs, Android phones and tablets, Apple phones and tablets, and anything else where they think it’s viable. Within their own private ecosystem, Amazon is making a real attempt to let you read your book everywhere you want to.
Adobe (the makers of such beloved software as Adobe Acrobat and Flash) has also created a DRM system. They’ve licensed it to a lot of people. The Sony eReaders use it. I think the Kobo uses it. There are various readers on various smartphones and such that can decrypt these files. The Nook can also decrypt files using Adobe’s system.
This is nice because more and more libraries are lending eBooks and they’re using Adobe’s DRM to do it. So the Nook can read books from these libraries and Kindles can’t.
For an example of the confusion all of this can cause, think of Apple’s iBooks app. iBooks uses EPUB files, but it uses Apple’s FairPlay DRM. So even if you get an EPUB from your library (assuming it was encrypted with Adobe’s DRM), you won’t be able to read it with iBooks. Even though they’re both EPUBs. If it weren’t DRM’d, you would be able to read it with iBooks or a Nook or a Kindle (after a brief conversion) or printing it out for that luddite feeling.
So, it’s DRM that limits your options and not file format.
There’s a little hope, though. DRM is inherently broken. If someone sells you a book, you expect to be able to read it. So you have to have SOME way to decrypt it. The DRM systems use the sneakiest methods they can think of to hide this way from you, but enterprising people have figured out how to expose it for the major formats (except, perhaps, for Apple’s FairPlay). Breaking DRM is illegal in the United States, however, so don’t ask me how to do it.
Also, I take a bit of hope from the music industry. Music downloads (on iTunes, for instance) used to be DRM’d as well. But, eventually, they realized that it wasn’t cutting down on piracy and was making it harder for consumers to play the music they bought (so they’d just go pirate it instead). Today, all the music you can buy on iTunes or from Amazon’s MP3 store are unencrypted. I expect the publishing industry to learn this same lesson in the next decade.
What does this mean for you vis-a-vis Kindles vs. Nooks?
If you buy something from Amazon’s Kindle store, you can only read it on a Kindle or with one of Amazon’s Kindle apps.
If you buy something from B&N’s Nook store, you can only read it (as far as I know) on a Nook or with one of B&N’s Nook apps (I know there’s an iOS one. I don’t know if they have more.).
If you want to buy a book from a third-party store, you’ll probably be able to read it on a Nook (it will probably use Adobe’s system) and you probably won’t be able to read it on a Kindle (unless they sell it unencrypted).
If you want to borrow books from your local library, you won’t be able to read them on a Kindle and you will be able to read them on a Nook (or a Sony reader or anything else that uses Adobe’s scheme).
If you have an unencrypted book in one file format, you can convert it to whatever format you want and read it just about anywhere. Project Gutenberg is a great source for public domain books in all of the major formats.
And that’s the skinny on file formats and DRM.
But the weight doesn’t bother me (still weighs less than your typical hardback book). It slips easily into my purse without getting in my way because it’s very thin.