Hey I just re-read this thread and I missed a point, I thought the original post was a question comparision between a 19dBd Yagi and a 24 dBd para... RATS.
Ok so two different antenna designs which have the same gain will both be operating with (very roughly) the same power level. Sort of.

Front-to-back ratio; the ration of signal at the peak gain point (0 degrees) as compared to the back (180 degrees), though a truer comparison would be from peak gain @ 0 deg. to everything from 90 deg. to -90 deg. the whole 180 degrees behind the antenna. A good FTB number is -25 to -30 dB down from peak gain; it's no big deal except in interference with users on the same frequency nearby; your signal could be picked up in the opposite direction thereby defeating the purpose of a directional antenna in the first place.