There is no wrong answer. People will tell you to use condensers of all kinds, and thats good advice, but so much of recording an acoustic guitar depends on the room you're recording in, that unless you're in a very appreciable space you'll be fine with any mics that aren't total junk.
I'm currently recording with a pair of Behringer C-2 microphones. They're more than adequate, and most all of the anomalous effects I'm getting in my recordings using them are not only easily fixed with EQ, but I'm quite capable of determining they're due to mic placement and my use of a less than ideal room.
Don't waste money on expensive mics to record with. A single large diaphragm condenser will sound perfectly nice on its own, with minimal mic placement. If you want to get more advanced and have more control over your mix and so on, then use a stereo matched pair of small diaphragm condensers, and experiment with mic placement.
Neither is a better approach. The latter sounds better, but its much more work, and if you have a full mix its not needed, the guitars nuance will only be crushed by everything else anyway. You may as well use the single large diaphragm in that situation and deal with how much easier it will make things overall, not having to worry about phase or stereo image.
On the other hand, using a sparse texture, your guitar handles a lot of frequency range, and you'll want to be able to adjusts the amount of overtones and detail you capture with your neck mic, vs your 'body' from the bridge mic. In those cases, use your stereo matched pair.
If you can only go for one of the two, go for the large diaphragm. It'll handle vocals and a number of other things more capably than small diaphragms will, making it more versatile at the expense of being able to use stereo micing techniques. It will still sound lovely for the vast majority of purposes.
For reference, I've recorded using multi thousand pound setups, and it really doesn't jump out as a massive improvement in SOURCE sound, to use expensive mics. You can achieve, with judicious mixing, a more polished end product, but the good mics only make that easier, as you can get comparable results with cheap mics if you're willing to simply be more careful with your mixing.
Thats not to say that on a professional level these things aren't worth it of course, but if you were a professional, you'd already know why and wouldn't need to ask us about this, so its really a non-issue.
My large Diaphragm condenser is an Essentials BM-1. My interface is a Presonus Inspire 1394.
The main weaknesses in my recordings are that I pick up noise from the immediate area, including elcetricals, and that simply put my skills are in development. None of the equipment is at fault there, and upgrading to really good gear would only partially solve the problem, if it affected it at all. All the gear I mentioned just now, which is 3 mics, an interface, a mic stand, and cables, cost me roughly £270. You don't need to spend a lot, just make sure you spend on what you really need.
Note - All these mics are available from Dawsons.
