I don't think you can really draw any useful information about how much people prefer each device for reading by comparing the average number of books sold per

I don't think you can really draw any useful information about how much people prefer each device for reading by comparing the average number of books sold per device. Everyone who buys a Kindle buys it to read books on. Some people who buy iPads don't care about e-books at all. That doesn't say anything about its capability as an e-reader, it just means that the device appeals to a wider audience.