I would suggest if your diving in 20 to 30ft of water only then a reel is best for that situation, but, a float line is good for someone who is inexpierienced and may feel more comfortable to let go of there gun and swim to the surface. A float line is good for several reasons, diver flag for alerting boats, float to hang on and rest, shoot and swim to surface without gun, etc, etc. The reel is good for ease of use, no line to get tangled in, streamlined approach, etc, etc.
Scuba diving is different and you should freeshaft so there is no entanglement of line around areas such as tanks or regulator which can be deadly if you spear a decent size fish, this could also get wrapped on structure or reef and not be able to free one's self. I would go with a reel, fish are not big at all in 20 to 30 ft of water.