Excellent X/Y stereo SDC cardioid mic for field use? Schoeps X/Y CLONE?
The above linked recent thread winds up discussing MS verses XY... start reading from about post #18 and beyond. I'm a long time XY fan, but this thread has really enlightened me to MS.
Based on all this, I'd think that an MS pattern might be best. The "mid" mic will focus directly on the subject, and the "side" will grab room ambiance... then later you'll have the ability to vary the levels of the subject verses the ambiance... and in a small living room, this might be a very important degree of control to have.
If the room is really small you might want to use a supercardioid as the mid mic to really zoom in and avoid any weird close reflections. Experiment with the position, but I'd say set the mic at the same height as the fiddle and aim it directly at the fiddle... at least as a starting point. Make sure the mid signal sounds really good because you will likely rely heavily on it.
AB... yuck... AB never works for me... yes, it yields a "euphoric" sense of width / spaciousness, very unrealistic though, and ultimately very "vague" and inaccurate sounding. It may provide more "ear candy" for the inexperienced recordist, but, at least for small arrays / small rooms anyway, it just sounds plain goofy... the transients and detail become "smeared" etc (time arrival differences plus phase issues, etc). This is why I've been Mr. XY for a very long time...
But MS seems to be the way to go for many or most situations, especially if the source is a single point (as in one soloist) where the mid mic can really zoom straight in and capture it nicely with perfect accuracy.... and then the ambiance (which might be good or bad) from the "side" mic can be blended in per taste.
Also, an omni in a small room... bad idea... in my opinion.
I've done a lot of recording in mostly smaller rooms and very small venues... I rarely ever pick up an omni mic.... I can't remember the last time I even touched an omni mic. Any time I've ever tried them, the results were horrible... huge, unfocused, overbearing low-end frequencies... a rumbling, booming mess... due to the small / poor rooms I'm often dealing with. I almost always use cardioids or super-hypercardioids. And now that I plan to get way more into MS recording, a good figure 8 will likely be in my back pocket all the time now too.
Also, as you probably already know, in general, small diaphragm condensers (SDCs) have a more even frequency response off-axis than large diaphragm condensers. Within the SDCs pick-up field, you'll grab "all" the frequencies, low to high. With many / most LDCs though, in cardioid pattern for instance, the pick up field of say 8k and up may be a LOT smaller / tighter than the pick-up field of say below 1k... this means you'll be "zeroing in" on a small area (grabbing all freqs) while picking up a lot of extra low end from a much larger area. In other words, many LDCs are like supercardioids from 8k and up, but closer to very wide cardioids or nearly omnis in the lower frequencies. In an untreated room (like the average living room etc), this is not really a good thing... typically.
So, in SUM... I think if I had to make one single recommendation based on all this, to record a single soloist in a living room, I'd say use good SDC mics in an MS pattern. I don't think you can go too wrong this way.