What specific frequencies are you interested in? For GMRS (462Mhz), I highly recommend the Nagoya NA-771G or the Nagoya NA-701G. Get them from buytwowayradios.com. The long variant does work well, I have tested it and confirmed this. I can't say which is better, because I've not been able to test them again each other, just again other antennas.
The 15" version is good, probably not so much from its electrical properties, but rather from its height. It is above your head, above other objects and just makes that last stretch of audio come in clearer, if you need it. You'll will much prefer the short 5.5" version, however. The 15" whip is just in the way except for all but the most extreme of situations.
Like I said, I can't say if the 15" is better than the 5.5". But I can say, they are both good antennas.