Having just obtained an iPhone 6 plus and retired both a Samsung Note II and Galaxy S4, I will give my 2 cents. I debated a long time on whether to upgrade to the Note 4, Galaxy S5 or go with the iPhone 6 +. I have a friend who had the Samsung Galaxy S5 and loves it. I loved my Note II not so much the S4. My son loves his Note 3. I have always had Samsung Android phones up until I purchased this iPhone.
Why the change? I have two Mac laptops and an iPad Mini. I like the way they "get along". I can attach the iPad Mini to one of the laptops and move stuff like pictures or videos around. iCloud works with all my apple devices.
I am now very use to OS X and iOS, so making my phone match was a no brainer for me. Less to learn and remember, more compatibility, can get text on all of them and the other new stuff I haven't had time to mess with yet like handoff.
And did I mention better malware control? I never had any issues on my Note II but I ran Lookout on it and I was a very careful downloader of apps and what websites I cruised.
Plus I have been very impressed with the iPhone 6 +. Smooth, fast, very good voice quality (I always was a bit disappointed with voice quality on all my Samsung phones during a call.)
If you have nothing else that is apple - laptop, tablet, then the Galaxy S5 is an extremely nice phone. If you have other apple products the cross compatibility is awesome. IMO