Honestly, the cheap-ass tube micpres like the Behringer just add a bunch of distortion to your sound. Most people "in the know" seem to agree that tubes aren't a big deal when it comes to micpres.
To your point about using one at all, whatever gets you a moderate input signal. Which for an acoustic guitar, will require a pre. If you don't use a pre, you'll have to boost the signal a ton in your DAW and you'll end up with a lot of noise.