Doesn't matter. Use primaries (CR123A) if you will be using the light infrequently. Use rechargeables (18650) if you will be using it daily, or very frequently.
Since two batteries and a charger costs upwards of $30, consider this: will you be using the light for more than ten hours at its brightest setting? (at $3 a pair of CR123).
4sevens sells batteries and chargers that will work for your G5. Buying from them will make things easier if you have warranty issues later on -- since you're using all kit you bought from 47s.