In .50 cal, either will work.
For calibers larger than .50, ffg is usually preferred, with the preference being stronger the larger the caliber of the gun.
For calibers smaller than .50, fffg is usually preferred, with the preference being stronger the smaller the caliber.
ffg burns slightly slower than fffg and has a slightly lower peak pressure. However, equivalent amounts of ffg and fffg will release very nearly the same amounts of energy in total.
ffg is preferred for the larger calibers because the slower rise time is needed to allow the usually heavier projectile time to accelerate.