My 2 cents -
I bought a 3GS in Aug 2011 (2 years after its release). That was my first iPhone.. Its still with me but runs very slow. Am only using it as a secondary phone these days for my personal number. My primary number (provided by my office) is put into BB z30 (as company has a tie up with a service provider and i get free unlimited data package)
I am looking forward to buy a 6s this September.. For me, the "s" series seems a more reasonable buy as it holds the most advanced features in a given number series.. Eg. 3GS, 4s, 5s and going by the rumours of 6s, its something worth purchasing and owning for atleast 2 years (till a 7s comes up).
Also in a "s" series, in addition to features enhancement and improvements, Apple gets a chance to rectify all shortcomings and issues found in the main phone..