I prefer adding polynomials horizontally because its quick to just group and add like terms. Plus you read things horizontally so it seems more natural. if you add them vertically you have to line up the like terms and place a zero where there are none.
Horizontal
5x³ + 2x² + 6 + x³ - 6x² + 3x - 4 = 6x³ - 4x² + 3x + 2
Vertical
5x³ + 2x² + 0 + 6
+ x³ - 6x² + 3x - 4
------------------------
6x³ - 4x² + 3x + 2
