Typically, it is not a matter of AB vs. BA (the correct order is AB), but a matter of which direction is the A stroke written; is it left-to-right, or right-to-left?
There are numerous sources that claim it should be left-to-right.
However, there are other sources that claim it should be from right-to-left; like a more (but not completely) horizontal katakana ノ.
Also from kakijun.jp:
This page explains more about each of these two theories, and a "conclusion". It's all in Japanese, so for those who can't read it all, here's the gist of it:
- The left-to-right direction originated with textbook publishers. Before 1992 (平成4), they were split on the direction of the stroke. Then in 2005 (平成17), they settled on the left-to-right direction. This is the direction that is taught in elementary schools.
- The right-to-left direction is due to the fact that the katakana ヒ is derived from the right side of the kanji 比 (not sure where you saw that it is derived from 世), where the stroke order of the A on the right side is written from right-to-left:
So the "conclusion" is that typically you write it left-to-right for 硬筆 (writing w/ a pen or pencil), and right-to-left for calligraphy (書道). But it's OK to do whichever is more natural or comfortable for you. I guess you could say that of kana/kanji in general, but even if you're a purist, this character is still ambiguous.
I personally do it from right-to-left like a flatter ノ, but that may be because it feels more natural to me as a lefty.