Android is much better operating system than iOS imo. I was expecting more from iOS 8 but they haven't really done much with it (I have a developer account so i've had the beta for a while now). It still has a simplicity aspect to it which is nice but nothing new. In terms of raw capability, power, customizability and features android has the edge.
I would go with S5 over the iphone 6 (although the phone itself seems pretty good, the operating system isn't that good). But if you want a simple interface, a high quality phone (not plastic) and with android's power, HTC One M8 is the way to go.
Before making any decision though, wait till iphone 6 releases and reviews are available.