That's a good question. I regularly get interested in e-readers but one key point in your question is affordable. Kobo readers [1] look promising. Especially the biggest one Kobo Forma (8") but it's far from cheap (280€).
In general, what deters me from e-readers is the lack of a nice way to annotate (with handwriting). I get the problem with EPUBs but it should be fine with PDFs.
Generally, EPUBs are a problem for technical books where I want the figures to properly match the corresponding text. Plus some figures are bigger than the screen (have to be so). And reading PDFs is a problem for the same screen size reason...
The best I can think of is Sony's gigantic DPT-RP1 [1] and it seems to have perfect handwriting support. But at almost 800€, I really don't call this affordable.