I recently traded in a Samsung Galaxy Tab 7+ for a Netbook, but when I did have the tab, it was decent for manga and artbooks. Like wizsi2k said though, don't expect to play much anime on a tablet PC. Although space wasn't so much of an issue with the Galaxy Tab (16GB onboard with a 32GB MicroSD), the processing power just isn't there, and to my knowledge, the 7+ had more power than the Kindle Fire does. My netbook now plays anime much, much, much better than the tablet did. It did play semi-decently when I was streaming using Emit (it's an Android app similar to the Air Video feature in iOS products), but local playback left a lot to be desired, even with standard definition stuff. And multi-audio/soft subs were a mess both streaming and local.
I don't know how much you are for/against reading ebooks and manga off of a computer screen (I'm assuming you must not care that much if you are looking into the Kindle Fire since it doesn't use e-ink), but I suggest possibly looking into a Netbook. They are nearly as lightweight as a tablet would be and much more functional for just about everything.