Let me give you a little insight as I've used 3 different phones over the past few months, two of them were the Galaxy S6 and the iPhone 5s.
First, the iPhone 5s is old, and it is showing its age. I know many here will swear by its longevity and it really is an iconic design, however it is severely limited by its 1gb of ram in 2015 and apps would constantly reload while I used it. This isn't a problem that is present in the iPhone 6s, my current phone, and while present somewhat on the Galaxy S6, it isn't as bad.
Second, the screen of the 5s is TINY, but this is probably more of a personal preference and if you like small screens it probably isn't an issue. Personally I found the Galaxy S6 and iPhone 6S displays (both around the same size) to be perfect.
Third, and this is what made it a deal breaker for me, was the ABYSMAL battery life of the iPhone 5s. I could burn through the battery in 3 hours or less of screen on time if I'm just pushing it. This is essentially what made me switch. The Galaxy S6 has a much bigger battery, and battery life is probably on par with the iPhone 6s from my experience, which is a huge improvement over the 5s.
If I had to pick between the two I would choose the Galaxy S6, as having used both I felt it was the better phone overall, and this isn't even counting all the additional perks like quick charging, wireless charging, and all the extra goodies which you may or may not use. Regardless which one you pick, they are likely to be supported for the same amount of time (around 2 years) as apple usually provides support after they discontinue a product, and it is still being sold. Samsung as with all other android manufacturers gives you about 2 years of support as well and you shouldn't expect more.
If you are going to be using this phone for a long time, you'll probably be better off with the S6 and the top of the line specs (android-wise) over a two year old iPhone. Hope this helps.