Given the newly announced iPhone 6 screen sizes:
iPhone 6: 1334h * 750w @2x (in points: 667h * 375w)
iPhone 6+: 1920 * 1080 @3x (in points: 640h * 360w)
If you prefer macros here are the ones, that you can use to differentiate between the iPhone models. These are based on the point values.
#define IS_IPHONE_4 (fabs((double)[[UIScreen mainScreen]bounds].size.height - (double)480) < DBL_EPSILON)
#define IS_IPHONE_5 (fabs((double)[[UIScreen mainScreen]bounds].size.height - (double)568) < DBL_EPSILON)
#define IS_IPHONE_6 (fabs((double)[[UIScreen mainScreen]bounds].size.height - (double)667) < DBL_EPSILON)
#define IS_IPHONE_6_PLUS (fabs((double)[[UIScreen mainScreen]bounds].size.height - (double)736) < DBL_EPSILON)