iPhone plus Kindle is a brilliant combination and one I use a lot. Most of the time I'll opt to read on my Kindle but for short bursts of reading or places where I might need to be discreet with my reading (ie when out with family/friends) it's easier to get away with reading on the smaller device!
Until I got the lighted cover for my Kindle, I used to read the iphone in bed - that's where the backlit device has an advantage. I suffer from insomnia so I can wake up multiple times a night when I will either read or write and I don't want to disturb hubby by putting a big lamp on. But having the lighted cover solves that issue completely now.
Happy reading