Right now, the iPad 3 is the best option because of its high screen resolution (no need to constantly zoom it to read smaller text captions). There are plenty of good comic reader apps for the iPad so that's not really something you'll have to worry about.
My biggest gripe with the iPad is storage. Highest you can get is 64GB ($699 Wi-Fi, $829 4G) and useable space is only around 59GB. I have to constantly juggle files around and delete/copy comics/manga to the iPad because storage is very limited. It's not like the "cloud" is a viable option with comics. While it's easy enough to use Dropbox or Calibre's server for ebooks (normally under 1MB each), the comics/manga I have are much larger (80-100MB per volume) and do take quite a while to copy.
So far, the only Android tablet that's been officially announced with relatively high resolution is the ASUS Transformer Pad Infinity 700 (1920x1200). No idea when it'll be released though, particularly with the news regarding Snapdragon S4 shortage.
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