Answer:
Overall vertical is visually better, if done correctly
it forces you to "line up" all the common exponents.
The disadvantage is that it usually requires re-writing the problem, and it takes up space.
most problems are presented horizontally, that becomes the issue to locate the common exponents.
in both cases the biggest issue is people forget
that when subtracting "subtracting a negative is like adding a positive"
-5x - (-8x) = 3x [that is a positive 3x]
or:
-7x
- - 10x
-------------
3x
everyone misses those eventually so you have to watch out for that in both methods
Explanation: