Okay, so loads of good points on here and may I say how nice it is to see people discussing the question without being reduced to petty arguing and... that horrible word.. f*nb*yism!
Which of these two phones should you buy? I asked myself that same question, and was torn between the two.
Some people really like the look and feel of the Galaxy phones - including me - whereas others like Sony's cool alloy and glass design - like me! I ended up going for the Z2, but it was real close. A guy in the office has the S5 and after playing with that, I really like it but for me the Z2 was the right choice.
Specification...
I really wouldn't even consider either one a winner. They both have excellent performance and the difference between them is tiny. You just aren't going to notice a difference between them really.
Camera...
both these phones have great cameras. Whichever one you choose, you WON'T be disappointed. Coming from an iPhone, you will have high standards - the iPhone 4S has a lovely little camera on it. Luckily though, both the Samsung and the Sony have really brilliant cameras. Yes, the Samsung has slightly worse low light performance apparently (I haven't tested it a great deal myself) but it probably just edges it for autofocus speed in good light. Swings and roundabouts.
Video...
I haven't spend much time using video on either phone, but I can say I was really impressed with the Sony's stabilisation. In comparisons, videos I took walking round the office and up the stairs were much smoother on the Z2.
Battery & charging...
the Sammy has a removable whilst the Sonys 3100mah beast is just brilliant. The Samsung is to have wireless charging, the Sony doesn't BUT it does have magnetic charger. (NOTE: It's funny, when the Galaxy S3 came out I remember the headlines 'it's going to come with wireless charging out the box!'... I don't ever recall seeing wireless charging for the S3. Then the S4 came out... 'it's going to come with wireless charging out the box!'... Nope, still not. And now they are saying it with the S5!).
I can't comment on wireless charging on the S5 but generally my experience of wireless charging has been pretty poor. On the other hand, I got a magnetic dock for ?10 for my Z2 and it is absolutely brilliant! Charges nice and fast, docks really easily, holds the phone at the right angle for me to watch TV on it whilst laying in bed. Perfect. I also bought a genuine Sony magnetic cable for about ?12 as well so I never have to worry about opening and closing the door of the phone.
The Sony charges up pretty quickly too, much faster than my HTC One M7 (what phone doesn't!)
Screen...well both screen are brilliant. You won't be disappointed with either, they are both full HD and both have really vivid colour so I'll leave it there for the screens.
Waterproofing...
again, both are waterproof, although the Sony will happily go with you in the bath whereas if you take the Samsung S5 in the bath, a Samsung rep will be standing over you , wringing his hands, and asking if you want to take it out the water yet?
Seriously, for general water-tightness both are fine.
Sound...
the S5 has a really loud speaker, but the Sony's waterproof stereo speakers definitely win here. I was impressed how loud my mates S5 was, but the Sony sound is better no doubt. Then there is the built-in noise cancellation and the free headphones. There is simply no comparison here, and this is one of the reasons why I went for the Sony. The sounds is just amazing. Combining the noise-cancellation and the great bass and full range of the free earbuds, the sound is just fantastic.
My only gripe is that I would like to have had flat, anti-tangle cabling and in-line remote. But you forgive them that when you hear them.
Other features...
well the Sammy has the fingerprint recognition and heartbeat reader. My mate has used the Sammy more than me, and so I asked his opinion. He said he was rather disappointed in the fingerprint reader as it didn't work that often, and what is the point if it isn't reliable? I haven't used it myself, and I wonder if it is just a question of technique, but defer to him here. He said the heartbeat reader wasn't very accurate either. But there is no doubting it, the Sammy is chocked full of little touches and features that the Sony just doesn't have. If you like playing with loads of little features, the Sammy is great in this regard. The Sony has digital audio out though, and has a Stereo microphone available for it - STM10. Although this works with any smartphone, it has been optimised for the Z2 and that makes a big difference apparently. I haven't got one yet, but as a keen bird watcher I plan to. I have noticed that the built-in microphone on the Z2 is excellent already though.
Camera side button...
Another little extra I really like on the Sony is the camera button on the side. It's so nice to just be able to squeeze this two-stage button at any point and it will take a picture. Again, I take a lot of photos and find this little button invaluable.
Then there are the fitness bands.
In favour of the Sony - great battery life, cheap, discreet and logs many aspects of your life. No screen though.
In favour of the Samsung - Although it's more expensive and logs less, it looks awesome, has a lovely screen, probably more useful and accurate fitness aspect. More potential too.
Memory...
Both of course have microSD slot, though the Sony has 3GB of RAM over the Samsung's 2GB. This doesn't seem to make any noticeable difference though.
So there you have it!
Clear winner? Well, there is a clear winner here - it's US! You would have to be a pretty petty or sad and confused person to buy one of these phones and feel you made the wrong choice; they are both so great.
I went for the Sony, as photos and music were important to me and I felt the Sony edged it here. You may like having all those little touches, a replaceable battery or take a lot of daytime photos, and so go for the Sammy.
Good luck either way with your choice, OP!
Oh, and remember the LG G3 has just come out now, and that has a QHD screen and TINY bezels!
R