Finding the ideal pickup for an acoustic instrument can quickly become a tedious, expensive, frustrating pursuit.
You can learn a lot online, but you'll soon fine that everyone has their opinion, and what works for them may not work for you. I often look to the quantity of positive reviews for pickups, rather than one or two opinions, as expert as they may be.
The best online resource then may be any sound comparisons you can find, ideally done in a controlled way (that is, using the same instrument, pre-amp, mic, etc - just changing the pickup). Another idea is to find players whose sound you like, and see if you can figure out what they use.
If I hear an acoustic instrument in a show that sounds particularly good, I do my best to ask the sound man (or the artist, if I can get to them) what kind of setup they are using.
Finally, be aware that a pickup is only one part of what can be a long signal path. The preamp can make a huge difference, as can eq and other effects. And of course whether you are ultimately going into an amplifier or house PA, etc.