i’ve got several e-readers myself, a 7 inch Bigme, a 10 inch Bigme, and a Kobo Libra Color. i bought the Kobo specifically to read japanese books on. since it has a handwriting/annotation feature, i can write the readings and/or a short definition of new words directly on the screen, and it’s worked really well. it’s easy to put non-DRMed books on there (although to be fair, i’ve never owned a kindle, so i don’t know how hard that is) and i’ve even put the entire device in japanese mode, so i have to learn to read the menus and stuff (i just checked, it has multiple flavors of chinese, and a bunch of european languages, but i don’t see a korean option). i haven’t found a good japanese to english dictionary option, but the jp-jp dictionary works fine. i did find a website with a tutorial about putting different dictionaries on here, but it didn’t work for me, probably user error (^_^)
the smaller Bigme i bought to read english novels on, and i chose that one because i own books from multiple places. if you’re someone who has ebooks from amazon AND google AND elsewhere, i would really recommend a Bigme or a Boox. the battery life isn’t going to be as amazing as a Kobo or a Kindle, but it’s worth it to me not to have to worry about where i get my books from, i can just choose whichever place has the best sale. i also put a couple of japanese ebook specific apps on there, and they both work fine. the Bigme default ereader app is okay, nothing amazing, but it also has a handwriting/annotation feature that works fairly well. the app doesn’t really know what to do with furigana/ruby in a japanese book, so i couldn’t use it, but for other languages, it’ll probably work fine. and of course since it’s literally just an android tablet, you can install KOreader or whatever other app you prefer. it is super easy to put books on this one too, and actually Bigme has a wireless import feature, so you can just point your browser to a certain port on your home network and upload the books that way, you don’t even have to bother with a USB cable.
as for eye strain and comfort, honestly, they’re about the same. the Bigme i have (B751C) has physical page turn buttons, which i love, and so does the Kobo (although those buttons aren’t quite as good, to me). the Bigme does have separate warm and cool light controls though, so you can choose how orange you want the screen. they both can get quite dim, so i find reading at night to be comfortable with both of them. the Kobo Libra Color is a bit bigger, because it’s got that sort of side chin thing, but obviously that’ll depend on what model you get. if i had to pick one to keep and get rid of the rest, i’d probably keep the Kobo. it’s battery life is just great, and i’ve already de-DRMed most of my ebooks, so i could just copy/paste them over and have my whole collection. if you’re leery of grey market ebooks, or if you already have a big collection spread over a few different stores, a Bigme or a Boox is still a solid choice, you’ll just have to charge it more often.
anyhow, sorry for the long post, i love e-readers. my eyes are terrible, i wouldn’t really be able to read without them. 
