since your were asking about what our preference is, i would choose the iphone 6. please note though that, to some extent, a person's choice of mobile phone is a very personal matter; the reasons that motivate one to choose a over b are unique to that individual.
if you can afford the iphone 6 or the galaxy s6 (i.e. price is not a consideration), i would suggest that you go to a place where you can get your hands on these phones and try them out. if you have friends who own these phones, ask them to show their phones to you so you can check things like: performance, user interface, response time, color, weight, feel, etc.
you may choose to gloss over the usual things like processor specs and RAM because, while these may be indicators of performance, the OS can play a huge part in your user experience.
also ask yourself if the apps you want are available on the phone that you are considering.
hope these help.
cheers, TS!