Yup- just what Track Rat said.
It's nearly impossible to try to do live mixing to tape at the same time that you're doing live mixing for the PA/front-of-house. That's a guaranteed ticket to both ulcers and low-quality results... The workload is insane, and the paranoia is worse- and you can never really hear what's going to tape. So picking off the PA feed is usually a disaster.
By far the lowest workload/highest quality result will come from setting up a stereo pair of mics right at your mix position, setting levels to tape, and then _forgetting_ about them so that you can focus on getting a good FOH mix for the paying customers. if the FOH mix is good, the tape will be as well.
This is one place where a stereo compressor/limiter like the Berhinger Composer Pro (or ideally something better) can save your bacon, and pay for itself in one night. You can use the limiter portion to keep those momentary peaks from burning a hole in your tape (or crunching your MD recorder), and still print good, hot levels. You can compress, too, if you want to- you may find it useful. But usually the acoustic levels at rock shows are pretty squashed just by trying to get the PA over the stage levels...
Set up the levels with 12-18dB headroom during sound check, set the limiters to clamp the levels *hard* at about -2dBFS, and then sit back and mix for the house! Glance down at your recording rig no more than two or three times during the night- if you're hard on the limiters, you'll need to pull down the gain at the mic pres a little. Every act is different, and it'll take a little practice to decide how much to sandbag on the headroom- but leave yourself _at least_ 12dB at sound check, because there's not a band in the world that doesn't play at least 12dB louder once there's a crowd in the room.
Your mileage may vary...