Dark Dragon 64, because I'm avoiding doing some work I might be able to help a little.
The Kindle Fire has a bright screen. Is colour. And you can easily pinch to zoom in and out. Of course there is the eye strain issue which does affect me if I read anything for too long, but then comics are more images then text. DC have released a handful of graphic novels that only work on Kindle Fire and will not work on the Kindle app for iPad or iPhone. Also, if you intend to buy music or apps you need a US credit card (but there are ways around this).
I downloaded a couple of Manga samples on to my Kindle Touch. This one from the screenshot was automatically displaying the page horizontally but it will depend on the publisher. I reduced the size of the image for the forum, but on my 17" screen that is still slightly bigger, and brighter than the actual Touch.
Where the text is written very small it was a slight strain to read. Many comics on Kindle are just a series of image files. Normally you can zoom in, but this one didn't allow me to. You can if you have PDFs. But overall I was surprised that it was very readable.
My recommendation is that if there are many Manga titles available then just get a Kindle Touch. If you want a graphic rich comic reader then either wait for the new Kindle Fire ( i expect 3rd quarter) or get an iPad 1 and download the comic app.