semi-acoustic guitar output mic or hi-Z ?

What brand or guitar/pickup? Taylor's new system has a low-z output, but most others are hi-Z. Of course, a piezo is VERY hi-Z so a preamp designed for piezos really helps. I like the Baggs Para-Acoustic as a low price option.

Of course, you'll probably get the best sound from the guitars if you mic them. You should record the pickups as well and send them to the PA rather than the mics. If that's not an option and the PA is not too loud and the crowd is pretty quite (a lot of ifs) a single cardioid condenser mic with a broad pattern in the middle might pick up a lot of the acoustic sound without too much logistic fuss.

GentleG wrote: I'm to record a live session of two singers with semi-acoustic guitars.
The session is in a pub, very small stage, with the audience right next to it, about 100 people. There is also a small PA. So I want to minimize the amount of mics, to prevent feedback and unwanted pickup of chatter.
Therefore I want to use the electrical output of the semi-acoustics. (the acoustics guitars do require a 9V battery to operate) My question is: what kind of output is this? is it low-Z mic? or hi-Z guitar?

It's NOT low-Z mic. Use an "instrument" or "Hi-Z" input on your pre-amp, or go though a DI box into a mic input. The point is that electronic guitar pickup units produce an unbalanced output, usually at a low to medium impedance at at a medium level. Unbuffered piezo pickups need really high-impedance inputs, as Bob said, but anything that requires a battery is not one of those.

Don't rely on the pickup output alone, even if that is all that is going through the PA and it sounds OK-ish live. You get a surprising amount of direct acoustic sound from the guitars in a small venue, and the ear is quite good at believing there is a favourable balance of acoustic to pickup sound. When you listen to the recording, you only have the recorded sound, and if that is only the pickup, you will be disappointed. Under these sort of conditions, I always take a close mic (12") track as well as the pickup. If the FOH sound allows it, I will blend in some of the close mic in the live sound, otherwise just record it. In mixdown, it's the other way round - I'll start with the mic track and blend in just enough of the pickup to round out the sound without the listener being aware that some of what he is hearing is from the pickup.

You could try the MK012s in cardioid, but the 57 is a safe bet.

Good luck - interesting assignment!