Capacitors vs buck/boost converters
Use a current regulator.

A voltage regulator won't do as LEDs change their resistance when they get warm and thus needs different voltage to get the same output from them. If you use a resistor you will just burn Watts and make a nice oven, even at 700mA.
current regulators are the way to go, don't cost too much and will be most efficient as you don't need any resistors and.
And if you want to be very efficient, select a switching current regulator...
It would be also a good idea to have the input voltage as close as possible to the LED(s) voltage.