in datasheets why measure of light output of power LEDs are given in lumens (luminous flux) but standard 5mm LEDs are given in mcd (luminous intensity)?
Luminous flux measures the total visible light emitted by the LED. It's a useful spec if you're going to use the LED to illuminate a room.
Luminous intensity measure the intensity of visible light emitted in a particular direction. It's a useful spec if you want to use the LED as an indicator and need to know if it will be visible against a bright background, or how bright it will appear when viewed directly, compared to other LEDs, possibly of other colors.
Since high power LEDs are often used to illuminate other objects, while 5 mm LEDs are often viewed directly (for example, when used as panel indicators), it makes sense to specify them differently.
Which type is more efficient?
You'd have to compare the output luminous flux (or intensity, depending how you will use the device) with the input power. Since the two devices aren't specified equivalently, you might have to experiment on samples of each type to find out.
If forward voltage is the same (same color) does that mean light output of 17.5 of 5mm LEDs equal 1 power LED?
Only if the efficiencies are the same.