iOS and Android are functionally very similar. iOS is a little more newbie friendly and much more stable as far as software goes. Android offers more flexibility and some different design choices. Personally, I prefer Android myself and stick with it for my phone though I use a mix of Android and iOS for my other devices as I like what each OS brings to the table.
Galaxy S4 is a rockstar phone that feels good, has a nice screen, and includes SD card support - a feature sorely lacking in many phones. So your friends aren't steering you wrong there. Definitely try the iPhone in the store though. iOS is laid out a bit differently and some people vastly prefer it. For me the phone is to small (but I use a Note 3 so obviously I'm into monster phones) but the OS itself would meet my needs as a phone if I had it.
If you tell us a little more about what you plan to do with the phone, we might be able to give some more specific advice on what kind of phone would be a good fit for you. Also, be sure to check out Android Central if you decide to go with an Android phone. They can give tons of good advice on which model is right for you, since there are so many choices.