With regards to the 36 month contract... I don't wait for my upgrade to change phones so something like this is nice as I get the new phone and I'm not paying an arm and a leg for it. I'd happily pay R350 per month or R7000 cash for a used one but will not spend R490 per month on a contract. I just don't like the idea of a high monthly payment.
And now to the android phones... I've had many Droids and iPhones and this is what I feel:
In terms of 3 years, that's the max I'd keep any phone. The S2 is comparable to the 4 even now and so is the S3 to the 4S. The android may be marginally slower but not too visibly. The S4 was bad though and the iPhone 5 will be faster... S5 was a massive improvement
IMO, the choice of OS is based on personal preference... iOS is very smooth, but so is Android on the S5, S6, P8, P8 Lite and so on that I've had in the last few months (I've had 30 to 40 phones in the last 2.5 years).
One thing that has always been better with Apple is the hardware quality and lack of bloatware (looking at Samsung!). There have been some really good quality android phones in the past few years (HTC One range) that could rival Apple and the bloat seems to be becoming better
The last generation of Samsungs show this - the S6/Edge is a massive improvement over the previous generation S5. The hardware quality is finally up there with Apple! Bloat has also reduced significantly, although carriers do still load a bit of crap on them. The software still needs more optimisation to be on par with iOS 9 but it's definitely getting there...
To me, on flagships, the specs don't count much anymore. I'd recommend choosing the OS you want and then grabbing the flagship you think suits you best within the OS. IOS and android aren't so different when flagships are compared - it's mostly personal preference now
The latest iPhone I've had is the 5S (had 2) but I'm getting a 6 Plus later today if all goes well and will post my thoughts and compare it to my S6 Edge
PS, I'm not a fan boy for any of the operating systems - I switch all the time!