Reading magazines on a 7" tablet is probably not the best experience. That said, you would probably want to go for the Nexus 7, since it has a much higher resolution display (66,66% more pixels!). The aspect ratio on the Nexus 7 is also somewhat better than the two others for reading magazines, it's relatively wider.
Also for big color magazines in PDF the somewhat faster processor of the Nexus 7 is desirable. Flipping through pages in big color magazines sometimes results in a couple of seconds loading time.
I'm not sure it is easy to install flash on a Nexus 7 though, since it doesn't come with the AOSP Browser but only with Chrome which doesn't support flash. You have to install AOSP Browser (or maybe some other browser that supports flash) in order to use flash.
Pluses for the tablets:
+Higher resolution display (and arguably better aspect ratio for magazine reading)
+GPS and Compas (could be helpful with e.g. for Google Skymap)
Galaxy Tab 2 7.0:
+Amazon ecosystem (but no Google ecosystem unless rooted)