Hey man. I used a galaxy s3 for a year, then an iPhone 5 for a little while, now currently on galaxy s5. My girlfriend and many close friends have iPhone 6's, 6 plus, 5s, etc. (most every teen/college student not into technology does). I'm also an app developer and have made apps for both platforms.
From a normal usage standpoint, each phone will do the basics great. Phone calls for the 6/6plus and top android phones will be the same. Texting the same. For the main apps out there you might use, such as twitter, instagram, Skype, whatsapp, vine, Facebook, tapatalk, snapchat, all of them will have the same features for both. There will be a difference, such as certain game makers will only put their games on iOS, but that rarely happens anymore.
The main differences are in the actual hardware and OS. For the iphone6, you finally get an iPhone with a 720p(it's a little higher than that, but not 1080p resolution) resolution. Androids have been doing this since late 2011. The iPhone also has 1gb of ram, compared to the 3gb of ram in the s6, and even 4gb of ram in other android handsets. The s6 has a really nice 16megapixel camera(similar to the one found in the Galaxy Note 4, so it is already praised). The iPhone also takes really nice shots, but at the end of the day is still 8megapixels. The processor in the s6 will also outperform anything apple could throw at it. The s6 also has built in wireless charging and samsung actually makes all of their hardware(sans camera on the s6) so they optimize it all cleanly.
When it comes down to it, it's not all about the numbers and specs, but I just wanted to lay some things out there for you to see that Apple will ALWAYS be behind when it comes to actual hardware. Apple does a phenomenal job in optimizing iOS to run really smooth on their slower processor and less ram. The thing is, iOS doesn't even have true multitasking. The top android phones are just so much more powerful.
As far as the interface, Android offers a heck of a lot more customization, but this part is strictly up to you. If you don't like how something looks or feels on the android phone, there are 3rd party apps that offer a different look and feel. If you do a lot of work on your phone, such as transferring files between your computer and phone, the iPhone is a pain in the ass. Say you download a file from the internet on to your phone. On an android, you can go into a file browser, and either open the file, move the file, delete it etc. On an iPhone, if that file doesn't natively open with an app you have installed, the file is stuck on your phone. iPhones are just so locked down it's annoying.
I advise going to the store and playing around with both of them, but I personally will never use an iPhone as a daily phone, even though I will always have one around to keep on for developing. The ONLY thing I ever thought the iPhone 5 had better than the s3, the iPhone 5s had better than the s4, and the iPhone 6 had better than the s5 was the nice premium design and the front facing camera. Both of these are fixed on the s6, so it's hands down a clear choice in my opinion.