I have a Kindle Oasis that I just got a few weeks ago, they had the price reduced and I have to admit I do love it. I still have my old Kindle from years ago but it was too old to support Japanese hence the new one. I think the Oasis does really well for both Japanese text and manga. I’ve downloaded a few manga on to it to read and it is really good. Sometimes furigana can be hard to read but I was sort of expecting that anyway.
I was considering getting a Kobo Forma which is probably the Kobo equivalent to the Oasis only it has an 8 inch screen I think compared to the Oasis 7 inch. Part of the reason I went with the Oasis in the end was because it was on sale and I thought the Forma might be just a bit too big for me but I do have small hands.
One good thing about all Kobo devices is that you can use Overdrive to borrow books from your local library (here in the UK anyway) but you can only do that with US Kindles. Not sure if that’s something you would want.
I think in the end it comes down to your own specific needs, I’ve just seen @ChristopherFritz post and I have to say I would never consider a 13 inch e-reader as I wouldn’t be able to comfortably hold it for any length of time.
Some very bad photos, sorry the lighting isn’t great.
I keep my Kindle logged into my Amazon Japan account and I have a Kobo UK account for English books. It’s not just as handy but it means I don’t have to keep switching Amazon accounts.