Hello Phil in AZ! Let me try to decrease confusion a bit. First, there are as many ways to record acoustic guitar as there are to paint a car. Consensus generally supports the use of condensers over dynamics, but that is not universal. Ribbon mics, a subcategory of dynamics, seem to be an exception, and ribbons have been used for many years on acoustic guitar with success. There are some cheaper ribbons now available from China, but they would be the only ribbon option in your price range. I would say, go with condensers for the most quality and versatility in your price range.
When using condensers to record acoustic, most people use one, two, or three mics. That can be any combination of small diaphragm or large diaphragm mics. The most common configurations are:
1. One mic on or about the 12th fret, close mic'd
2. Two matched mics in coincedent (X-Y) stereo configuration, backed off a bit. An additional mic, above the player's right shoulder, pointed down across the bridge, is optional. This will tend to pick up more of what the player hears, as opposed to what the audience hears.
These formats can use either small or large diaphragm mics in any combination. It's a matter of preference, and there are as many opinions as there are cats. In general, SD mics produce good high end detail, and cut through dense mixes well. They are noisier, on the whole, and can be excessively sensitive at times, which can be an issue with finger and pick noise. LD mics often produce a richness that adds depth, especially in solo guitar recordings. Both are viable, and both have been used for years with success. You don't *have* to do it any particular way. It is useful to have the omni option. In some cases, especially if you have a very good room, close mic'ing with omnis can reduce "boom", excessive low frequency output, produced by the proximity effect of directional mics. That, also, is up to preference.
So now the big question, how to spend the money? A lot of people are suggesting MXL603. If you go that route, I would suggest a matched pair of MXL 604's, which are very similar to the 603's, but with omni capsules in addition to cardioid. Matched mics are helpful for making balanced stereo recordings. A pair of 604's are about $200, but there is a lot of markup there. I can get them for $130 the pair from Guitar Center, out the door, so negotiate a little.
The Pelusos mentioned above I have never used, but a lot of people whose opinions I respect here recommend them. Personally, I would spend the $500 like this- A pair of Studio Projects C-4's, which I prefer to the MXL mics for most things. They come with a flight case, shock mounts, and cardioid and omni capsules for $300. I would spend another $160 on
an AKG Perception 200, a large diaphrgm mic that comes with a hard case and a shock mount. This will allow you to play around with all the mic combinations outlined above, and will be versatile for many other options for recording lots of stuff.
There are thousands of other valid ways, and this is just my suggestion- YMMV. Best of luck.-Richie
P.S.- You asked about Shure SM81. This is a well respected, mid-priced SD mic that has been a stage and studio standard for years. I can say nothing bad about it, except that for versatility, and as a learning tool, I'd rather have at least two mics for recording acoustic, preferable three. A pair of SM81's and an AKG C414B-XLS would rock, but that is about $1600 worth of mics.