I'm not very familiar with either one of these packages myself, but I believe the leaders in this category are Renewed Vision's ProPresenter on the Mac—which will meet your needs with the Advanced add-on— and MediaShout on the PC.
You could stick with PPT on the PC if you got either a Matrox DualHead2Go. Alternately, you could replace your graphics card with a dual-headed graphics card and use PowerShow to control multiple PPT shows over the two monitors.
Watchout is great, but it doesn't have a built-in titler and I think this would create a huge amount of pre-production for you in this case.
I'd also think you'd want a separate switcher downstream from the graphics computers if you want to feed live video.
Walter Soyka, Principal
Keen Live, Inc.
Presentation, Motion Graphics & Widescreen Design
RenderBreak: A Blog on Innovation in Production
