Excellent discussion and very informative. I am in the early stages of building a house and I've been researching the electrical options for my dedicated listening room. I'm certainly no expert but here are a few things that I think I've learned along the way. Please correct me if I get any of this wrong (I'm sure you will!).
One of the advantages of a 20 amp circuit is that code calls for the wiring to be larger. This should allow for a smaller voltage drop from your breaker to your outlet than a 15 amp circuit. If the service panel is close to the outlet this isn't an issue but in cases where there is a long run it could make a difference. I don't know how much difference a few volts make but I understand that higher voltages are better. This would indicate that for the marginal extra cost a 20 amp circuit would be better even if it won't be supplying high wattage components.
A few decades ago I was powering a pair of Large Advents with an Adcom GFA 555 and the lights would dim during bass transients. This was in a relatively modern house (70's vintage) on a 15 amp outlet. I installed a built in cabinet for the stereo equipment and included a new 20 amp circuit. The dimming stopped.
If you run a large Class A amp it might use more power than you think. I bought an inexpensive watt meter to measure my Krell KSA 300S and found that it uses about 1000 watts at idle and about 1700 watts when it hits the highest bias level. Fortunately I have a 20 amp outlet to plug it into (the guy who built my house was an electrician) but I would not want to run this beast on a 15 amp circuit.
At AXPONA several years ago I met Caelin Gabriel from Shunyata in their room at a slow time (really nice guy BTW). We talked for quite a while and I picked up a few tips - one of which concerned outlets. Caelin very candidly told me that Shunyata's outlets are great but if I wanted to be more cost effective a Hubbel 5362 was very good for a lot less money. I bought one (Amazon) for my 20 amp circuit and it is obviously better built than the contractor outlet. He specifically advised me not to use hospital grade outlets. These outlets are made of stainless steel which does not conduct electricity as well as copper. They are designed not to corrode in an environment where strong cleaning chemicals are used regularly. Since the medical instrument field is one of his big markets I figure he knows what he's talking about.
For the new house I'm considering a separate custom panel for the listening room similar to what Fremer did with his room (I would even use the same contractor for the panel). If anyone has done this I would like to learn from your experience.