It's also about what you need the pickup to do for you and your sound, your style of music.
For guys like me who play fairly loud Rock n Roll / Rockabilly type stuff, alot of the "nuance" of the double bass sound gets smothered when you are playing at club volumes.
So, it is a compromise. I like a fat, punchy tone that cuts thru a band with electric guitars, while still keeping a decent amount of "woody" quality to the tone, and provides that huge deep "pillow" of bass that drives a band.
For me, the trusty ol' Underwood does that very well, and so far has been the best at not being too prone to feedback either. It still sounds like an amplified double bass, which IMO, is "a sound" unto itself. A good sound.
The Underwood is very easy-going about installation and pressure also. As long as the pickup is just snug in the wing, it sounds good. I always use mine with just one pickup installed on the E side of my bridge. This gives a more "open" and deeper sound with less mids than using both transducers.
It also all depends on your bass 's character and your idea of what a good "sound' is.
I would still like to try a Planet Wing though, from what I have read it may just have the Underwood beat.