For upright bass, I'm using an Underwood transducer - like this one:
https://www.billyhydemusic.com.au/shop/index.cfm?action=view&id=6160 I run it thru an Alembic F-2B preamp direct into the audio interface, and the bass sounds much better than I sound playing it
I haven't had experience with any other transducers to compare, though. This one was under $200, and seemed to elicit favorable opinions from folks.
Actually, I just recently recorded a little improv "thing" with it. Its not much of a bass (or talent
) showcase - rather a demo for a synth controller that I build. But you can at least hear what the upright sounds like with that setup, running into a cheap M-Audio interface:
I haven't been down the acoustic guitar path yet...