I currently have a Nexus 5 that is running Android 5.1 at the moment. I have never had an iPhone. My upgrade is early next year. Nexus phones and iPhones come with the latest versions of their respective OS's. Google I/O 2015 will clear some details for the Nexus phone(s). WWDC will probably do the same for the next iPhone(s).
Read more:
How to decide between this year's Google Nexus phone or this year's iPhone?