I dunno. But just looking at the 12 new releases featured on my Amazon instant video front page, several are not available to rent that are available on iTunes, but not vice versa. This is more often than not the case with new releases in my experience.
Prices are still generally better with Amazon when it comes to purchases though. That's why I feel a combination of the two is the best bet, as neither is perfect for everything I would want. I find the Netflix/Amazon Prime situation to be the same way.