To answer the OP's inquisition: I buy whatever feels right at the time.
I'm at BN last Sunday and I notice a Terry Pratchett book on sale for $6.98. Pull up the Kindle app on my phone to check the price there and it is $14.99 for the Kindle edition. Guess which one I got?
I'm reading a fantasy series right now that is physically impressive in printed form. So massive I can just hear my RSI pleading for mercy before even picking the books up. Price is identical $9.99 for the Kindle edition OR the mass market. Went with the Kindle version: which I will read on my light and comfortable idol 3 that I can easily (handily?) wield without any major strain.
I guess I'm a pragmatist. All of the formats available to us nowadays (audio, e, printed) appear to have benefits and cons as far as I can tell and why should I limit myself to only one or two of them?