Something just occurred to me...I'm surprised I didn't pick up on this before.
When the subject is the Tanakh (OT), any perceived discontinuity or lack of "smoothness" in the text, resulting in a "hard text", is considered evidence of multiple sources since the editors ("redactors") would NEVER consider altering a text even when so-called "contradictions" are evident, (i.e. the documentary hypothesis--Wellhausen, et. al.).
In the NT, it seems to be just the opposite. When there are two text versions, the one that results in the "more difficult" reading is often considered the more likely original of the document author, since it is assumed that editors ("redactors") operate in the spirit of "cleaning up" the text, (i.e. Metzger, et. al.).
Funny how the "redactionist spirit" did a complete about face when BC became AD. 
I'm sure some can stick a wedge in this, but the assumptions of the two groups of text examples do seem to have opposing starting perspectives.
Odd.
