I get the vague impression that kindle is easier to buy/load books onto, but I've only ever used a (old-generation) Nook for about five minutes, so don't know. I definitely get the impression that the number of books available is much higher for kindle than for nook.

I find myself, these days, using the tablet much more than I use the kindle... if I were to get one device, I'd actually get a used ipod touch (or android equivalent), which I think are selling for about $50 -- it's like having a smartphone only without the "expensive" and "charge you every month" part. But that form factor may be too small for you.