Tablet? I don't own a tablet or an e-reader, but seems like an obvious question. A tablet would cost more, but would be more versatile?
Yeah - I'd go with a tablet myself. I've got a nice Android tablet that I picked up cheap from computergeeks.com before they shut down their online store; I think I paid $129.00 USD or so for it.
I had a particular set of specs I wanted - but mainly, I wanted it to have WiFi, GPS, and a camera - it ended up having that, plus two cameras (front and rear), plus the ability to drop in a couple of SIM cards (so, in theory, you can use it as a phone!).
I looked far and wide for something like this from the "big name" manufacturers of android tablets, particularly the Motorola Zoom and Acer's line of tablets - and I could not find one that had everything - most of the android tablets would have everything I wanted - except GPS - even for a tablet costing hundreds of dollars. I just don't get it.
Though maybe things have changed since then.
Anyhow - I read PDFs and other ebook formats just fine with it, plus I can access the web via wifi, read email, etc. IIRC, it has Ice Cream Sandwich on it - plus it also has USB OTG (so in theory I could do Android/Arduino stuff, though I have never tried it).
Today - you should be able to find a very capable and cheap Android tablet for around $100.00 with a capactive touch screen (don't get resistive, whatever you do!), camera, wifi, Android 4.0 (and up), with more than enough memory and speed to handle PDFs and everything else. Drop a large micro-SD card on it (32 gig or so, whatever is the max the tablet can use), and you'll have a great e-reader and everything else machine at hand. If you shop around, you might even get lucky and find one with GPS as well, if that is important to you (I had wanted GPS to allow me to use it as an "all-in-one" portable device that I could carry in my truck on a mount, and have GPS maps, plus MP3 playback - oh yeah, that's another thing my tablet has - FM radio pickup -and- transmit capability - so I could playback MP3s in my truck without needing a fancy audio hookup for my stock stereo).