Answer
The web should be vertical (the diagram on the left in the textbook).
Work Step by Step
The web should be vertical because most of the stress is in the top and bottom regions of the beam. With a vertical web, those regions have a lot of material. When supporting the load, one slab of material will be under tension, and the other one under compression.
If the web were horizontal, only the edges of the slabs would be under tension and compression, and there would be a lot of "wasted" material in the neutral layer which contributes weight, but little strength.
This is discussed on pages 232-233, and in Figures 12.8 and 12.9.
