Hi,
I am currently using a Samsung Galaxy S2 which has treated me very well and I have had no issues with the phone yet.
I am considering to upgrade, but would like to make an informed decision.
Which phone would you choose and why?
I have had almost all the galaxies, the new TABS excluded. On all of my devices I flashed CyanogenMod because the phone started going slow. If you don't mind that I would say go for the S5. If you want to stay stock for the whole lifetime of the phone, I would go with the 5S. It is cheaper than the S5 at the moment and I guess in 6 months time you will get better resale value if you want to change.
Also, I haven't noticed that the iphone 5 slowed down, so I wouldn't think that the 5S will either. I have noticed that my galaxies slows down over time if I don't go the CM route.
Things to keep in mind -
Samsung has some really nice integration with it's TVs and it was really cool to flip through my photo's when the parents were up. You can equivalent apps, but nothing so nicely integrated in my opinion. I also really like S-Planner, in my opinion the best android calender. There is also some other "cool" things, but then again I never really used them and I don't think it is worth the bloat.
You need iTunes on the 5S, it was really problematic for me at the start, but since I now buy almost all my music from there I don't really mind any more. I also had a big issue that I couldn't just use it as a USB device, but I think I only ever used my galaxy once like that. No-one I know walks with a cable around, so I don't think it is that bad. With the 5S you do limit yourself a bit, but ask yourself if you really need all the other features. If I didn't walk around with my Galaxy and replaced it with the iPhone I don't think I would have many situations where I would have wanted to use the other "open" features.
That said I will probably go for the Galaxy again, but my upgrade is only later in the year. Will see if there is no Nexus around the corner rather.