Hmm, I'm not 100% sure, as Sudden Leap doesn't really specify how the two are divided.
Generally I would say that it is fair to "split" the amount of distance you are leaping between vertical and horizontal distance in this case. So say you have an enemy that is 20 feet above and 20 feet ahead of you. You would need to move 15 feet forward And 15 feet upward to become adjacent to that enemy for your strike, so 30 total feet of movement would be needed, after which you would fall 15 feet.
A more strict reading of is that you can only high jump straight upward the indicated distance, or leap upward 10 or 15 feet and only 5 feet forward. This should normally be enough for most instances where you want to sudden leap anyway imo.
Leap, Long Jump, and High Jump all go out of their way to distinguish between horizontal distance, vertical distance, and combinations of both. The modified high jump that uses the long jump DC specified in sudden leap and other actions like fantastic leap doesn't make any mention of vertical or horizontal distance. This indicates to me that the distance is a generic distance that can be any combination of vertical and horizontal, thus defaulting to the standard rules on 3D movement as long as there's a clear path from the jump point to the desired location:
As with ground-based movement, moving diagonally up or down in 3-D space requires counting every other diagonal as 10 feet.