i have PP and my mum has a 6" one... so i had a chance to compare them.
One thing i noticed with the 6", is that it is significantly heavier (in its cover anyway) than the PP. The lack of scroll wheel is a big minus.
BUT as far as the easiness of reading goes, that one inch makes a biiiig difference. Not enough for me to give up the portability and light weight of PP, but if someones eyes are not the best (like my mums for example) i would def recommend a 6" one.