You're assuming that an iPad mini screen would be roughly as usable as a nexus 7 screen, but if the rumours are correct they would be very different. The Nexus 7 has a 'letterbox' widescreen aspect ratio compared to the iPad's chunkier 4x3 screen. Also the iPad screen would be 40% bigger. These are not small differences.
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You're assuming that an iPad mini screen would be roughly as usable as a nexus 7 screen, but if the rumours are correct they would be very different. The Nexus 7 has a 'letterbox' widescreen aspect ratio compared to the iPad's chunkier 4x3 screen. Also the iPad screen would be 40% bigger. These are not small differences.