Actually there are huge differences in reading when it comes to ppi. Tried prolonged reading on a Nexus 7, compared to the iPad 2. you'll find that reading on the iPad 2 is like reading without wearing specs, for short sighted person. The words are blur and not crisps enough.

whereas reading wise, when I compare nexus 7 with iPad 3. The difference ain't so severe. So I'm guessing above a certain level of ppi, normal ppl like me (us) will find it harder to differentiate between 2 devices with different ppi?

btw all these is under the condition of not zooming in on the reading materials.