The 4S has pretty much just expired for most folks, and it came out in 2011.If someone was to purchase an iP6 Plus today, is it likely it will still be usable, i.e. fairly fast, support updates etc in the next 1.5-2years?
The only issue with the 6+ is that 1GB of RAM pushing that huge screen. It did have lag/latency issues from the get-go.