What do you prefer for MS recording of acoustic guitar:
two LDC - or - LDC with SDC?
... and why?
I use AKG Blue lines for that - SDC. They sound better and the LDC mics I've tried (SP C3 and Rode NT2000). In general, SDC mics are cleaner off axis than LDC and MS is all about off axis.
You're talking studio rather than live stage performance, right?
It depends on the guitar, the player and the style of music. Also depends heavily on whether this is a solo guitar recording (maybe with added mono vocal), or has to sit in a mix (both spatially and sonically) with other instruments. The other factor is what sound the performer wants from his recordings, although this is more of an issue with jazz-style guitarists.
I've never mixed LDC as S mic with SDC as M mic, as I've always felt the sonic differences between the mic types would lead to an unsatisfactory result. For a long time I used a pair of Rode NT2-As for MS acoustic guitar recording, and they worked well on most guitars, players and styles. I've also used X-Y SDC pairs and stereo mics like the Rode NT4 in cases where M-S was giving too vague a sound.
In addition to a conventional MS or XY pair, I've more recently been putting up a stereo ribbon run in MS Blumlein (like conventional MS but the M mic is Fig-8 as well as the S-mic). The ribbon sound is quite different, sometimes too boomy and dark, but the configuration can be less sensitive to the player moving the angle of the guitar during the performance.
Basically you have to have in your armoury several methods of recording acoustic guitar that you are familiar with using, and bring out one or more of them for each new recording situation. It's not fair on the performer to experiment with unfamiliar techniques, unless by arrangement. This can result in a chicken-and-egg situation, of course, so sitting in on other people's recording sessions is also a very instructive exercise, noting what works well and what not so well.
Always take a pickup track in addition to any mics. It can come in very useful for blending in a small, delayed amount at mixdown, especially if the performer says he is not completely happy with your lovely acoustic mix.