If I was reading colored comics with any regularity, I’d almost certainly go for an Amazon Fire tablet, though there are Chinese tablets with very decent screens around, with higher DPI, these are generally nowhere as well supported as the Amazon tablets.
Chinese tablets often can be had with either Android or Windows (sometimes both), but good luck having any updates for Android, and Windows can be a little much for such cheap hardware.
While it’s true Fire tablets do not have the Play Store, their main limitation IMO is the weak hardware and lack of cellular connectivity. Of course they’re meant as in-home media consumption devices, and these days smartphones have massive screens anyway (they’ve been inching closer and closer to 7"), but I wish there was an option to at least drop in a SIM card and be covered in places where there is no Wi-Fi yet.
It’s also annoying that Amazon does not offer everywhere all of the models that are available in the United States: in this neck of the woods, the only tablet with a 1920x1080 screen, the Fire 10 HD, cannot be ordered.
With all that said, the price is hard to beat.
Kindles can display double pages side-to-side, so I’ll assume Fire tablets can, too.
I do read manga on my Kindle Paperwhite from time to time, and it’s a highly usable device, however, and despite high DPI, the 6" screen is a tad small for double pages.