Remember that the Amazon unit is a closed system and rely on you having a heavy investment in the Amazon universe, where a Nexus is an "open" operating system that once released on the hardware, you are on your own.
Amazon has the Kindle connect to the Amazon universe where you r content resides, books, movies, videos, music - purchased through an Amazon account.
Consider an iPad?
I ask about the iPad because there is a report released last Thursday that tell of malicious software / Trojan software infestation in up 1,743% in the past 6 months in the Google Android MARKETPLACE (I stress marketplace - some of it DOES get through though to your phones/devices) where as I don't think I can name a single infection for Apple.
But of your two choices, I'd go Android hands down. (But of the three Apple by a mile then Android then..... well anything else)