It sounds like maybe a stellar sound is not really required on this gig -- no sound check, end of the parking lot, playing closed position chords on a 12-string, etc. With all the compromises you're forced to accept already, it may be best just to go with whatever option is easiest.

As has so often been said, acoustic guitar pickups are all about compromises. I have Rare Earth Blends installed in both of my main acoustic guitars (Lowden and Larrivee), and I agree with Rick that they are the best things I've come across so far, when I consider the compromises I'm forced to make -- the quest for a reasonably authentic acoustic tone with no "quack" (I HATE the undersaddle quack!) and some ability to work the dynamics, being balanced against my desire for mobility on stage, a relatively simple setup, and the need for good feedback resistance (with the option to dial out the condenser mic altogether in high-volume situations). But I also agree with him that the improvement in sound over older soundhole pickups is marginal; this is not the Holy Grail.

For your upcoming accompaniment gig, if you have choices, I probably would not opt for the Duncan Woody. On 12-strings, you can often run into problems with soundhole magnetic pickups, which respond better to the unwound octave strings than they do to the wound strings, giving you kind of an unnatural sound (if you choose, you can go with nickel-steel strings to reduce this; the GHS "White Bronze" sets are one option). Since you'll be outdoors in a parking lot, my guess is that feedback will be less of an issue than it would be if you were in an enclosed space where all the sound waves bounce around; in my experience, outdoor gigs are somewhat more forgiving when it comes to feedback. So I would probably recommend sticking a mic in front of your guitar -- SM57, SM58, or really anything you've got (yeah, it restricts your mobility, but I assume you're not going to be doing Pete Townshend leaps playing closed-position jazz chords anyway). I'd probably bring the Woody along as well, and maybe have it plugged in just in case the mic starts to howl at you, so you can do a quick channel switch at the board and keep on playing. Also, one thing you can do to help with feedback is, if you have separate monitors, put the pickup through the monitors and the mic through the mains. If you'd rather stay away from a mic altogether, a contact piezo is another option (position it ahead of time in the privacy of your own home so you can find the "sweet spot").

I'm sure the gig will go fine; my guess is that neither your singer nor your audience will know the difference, no matter what pickup you use. Have fun.