I'm deciding on what iPhone to buy?

My three choices are: iPhone 5c, IPhone 5s, iPhone 6. Are there any real differences besides external stuff? I mean there all fairly fast right?

Yes they're both fast. If you have the money, iPhone 6 is the best choice between the three. Aside from being a new flagship, it has a faster Wi-Fi connectivity, NFC chip and new hardware which is A8 and m8 coprocessor and a bigger screen. If money is your problem, then 5c is the choice. Iphone 5c is cheaper than both two phones, goodlooking because it is colorful, but the internal was same from iphone 5. If you want a premium, good design and a smaller screen, almost same as iphone 6 but cheaper and much older processor than iphone 6 but newer than iphone 5c then iphone 5s is your way to go.