#1
Im about to buy a XLR plug to record on my mac, which sounds best when recoding guitar?? stereo or mono????
#2
what is the source?

unless its a stereo mic, then every track should be in mono.
#3
Guitar is basically a mono output instrument.
If you going to use microphones on an amp, you can use two of them for a stereo sound.
If you direct record the guitar, then mono. Later, effects can go stereo or multitracking by recording the guitar sound twice.