Is there a medium weight font between -systemFontOfSize: and -boldSystemFontOfSize:?

Starting with iOS 8.2 you can use:

[UIFont systemFontOfSize:14 weight:UIFontWeightMedium];

Calling NSLog(@"%@", [UIFont fontNamesForFamilyName:@"Helvetica Neue"]); prints all available font styles for Helvetica Neue, among them is HelveticaNeue-Medium which sounds like the one you want:

UIFont *font = [UIFont fontWithName:@"HelveticaNeue-Medium" size:14.0f];

If you want to make sure that the font also changes when the system font changes (eg. like it did with retina devices from Helvetica to Helvetica Neue) you could first get the system font and then take its familyName and pointSize to retrieve one with medium weight using + fontNamesForFamilyName and then + fontWithName:size:

Swift 2.0, xcode7.1

if #available(iOS 8.2, *) {
   titleLabel?.font = UIFont.systemFontOfSize(15, weight: UIFontWeightMedium)
} else {
    titleLabel?.font = UIFont(name: "HelveticaNeue-Medium", size: 15)
}