My boyfriend just recently went to an S5 after having an S4, a lot of people know have had problems with the S4, mainly with randomly shutting off and turning back on, and also "eating" the sd card, deleting all the data and being unable to read it, so there goes all your app data, your photos etc.
He has the S5 now and he absolutely loves it, though I personally hate it because it doesn't fit in my pockets nicely, I can't fit it in my bra, boot or a pocket if I were to go out riding on my own, and it's too bulky to want to have it strapped to my arm. It's just too big for me. That is my only complaint about it, that and it takes so darn long to charge, but in saying that we just recently found out that it does have a mode for when you are getting low in battery and can't charge it yet, it goes right back down to the most basic form (even black and white screen) and only allows you to txt/call etc, and it will supposedly last 24 hours on 1% battery, very good if you were to say go out camping or somewhere without power for more than a day, as he does have to charge it every night otherwise.
I have the iPhone 5, and I love it, have had it since October and not had a problem with it at all, my only complaint is having to charge it every night but I don't think any phones do last much longer than a day anymore. I like it because I can charge it while playing music through my car head unit, no other phone brands are supported through my head unit unless I use the aux cable, not handy when I need to charge it. I love the size, while it's big I can still fit in in most of my pockets, or comfortably on my body somewhere if I go out riding.
They have taken the iPhone 5 off the market however, so if you wanted to go in that direction your next best option would be the iPhone 5c, which is basically exactly the same, only it has been made cheaper due to having a plastic outer rather than glass. Or even the iPhone 5s if you wanted to spend extra, though I think a 5c would do just fine.
In your situation I would be debating between the galaxy s5, and the iphone 5c. I am an iphone girl, because of the size, and also because I find them so easy to navigate. I can use a galaxy, but I'll be darned if I can figure out and actually remember how to use it to it's full potential.
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms