It's dependant on so many variables it's almost pointless to compare.
The 6s looks good and I'll be tempted to move away from Android for the first time (Nexus 5).
I currently have a iPhone 4S (after previously owning an iPhone 3G) and I decided to switch to a 2013 Nexus 5 for now to try out Android for the first time (on a phone) as I was a little bit underwhelmed by what the 6S was offering.
Will give it a shot for a few months and see how I get on!