There's a jam session portion to studio auditions with samples. gearwire is more dependent on whether any of those mics are featured. They'll do some shootouts, but the low end mics don't get much love.
Baring samples you can just look at specs. SPL limits shouldn't be too problematic for acoustic guitar. Noise floor, frequency response, polar pattern, and other hints to what might or might not be part of the product.
SM81's are a common mention for your task. Not cheap, even less so across the pond. Sometimes you can get them used for a 2 fer, i.e. 2 used for the price of 1 new. And not specifically sold as matched pairs. Most mics on your budget will likely pale in comparison. But that really depends on your backend. Are you looking for a stereo mic for your iPod? Or something less cheap?
On that cheap side, you might consider a field recorder like device with mics. Zoom H2 comes to mind. Not two individual mics to be placed where ever, but it has 4 mics and other features.
Zoom H4 / H4n too. Lots of stereo options, most on a budget still kind of suck though. So again, what end result? How good is good enough? Best possible for a one off, that money might be better spent on studio time at a studio with better gear.
