aja wrote
What is the difference in sound of a 2x12 or a 4x10 cab(or amp) in general?
In general a 4x10 will be more accurate and have a tighter low end, while a 2x12 will have sweeter high end and a bigger, looser bass response. It's a moot point though as it really depends on the speakers in there and there are big differences between speakers, even different sizes in the same range.
Which would be better for classic clean sounds with slight breakup at times? ???
Depends entirely on the speakers. Also depends on
which classic clean - there are dozens of classics. Take an example of a sound you like, find out what was used and take it from there.
A useful resource for 12" speakers is the Proguitar site:
http://www.proguitar.de/AudioDemo/AudioDemo.html, especially in this case the speaker roundup (Blue Bull Dog, Greenback, Vintage 30, G12H, Jensen C12N):
http://www.proguitar.de/AudioDemo/CompareSpeaker/CompareSpeaker.html