They both use the same assortment of apps to read. The only differences are the higher specs on the Nexus 7 (quad-core CPU, which isn't actually much better than the dual-core in the Arc since most apps don't take advantage of extra cores - the Arc might actually perform better, in some cases. Bluetooth & NFC, a GPS and a few other extra sensors, probably better GPU). Due to the extra stuff in the Nexus 7, there will be more apps that can be installed directly from the Play Store (much of the stuff Play won't show for the Arc can be sideloaded, if you can find them elsewhere, however).
If you're in Canada and aren't buying it online, the Arc will be $39-40 cheaper for the same amount of onboard storage (at 16GB; haven't looked at the higher end prices lately). The difference goes down to $10 cheaper if you buy the Nexus 7 directly from Google Play ($199.99 for the Arc, $209 or $239 for the Nexus 7, depending on where you get it)