Hi
Photostage is most probably the easiest to use. Whilst allowing you to create a simple slideshow of images consecutively on a single timeline, or image track you can also insert video clips at any point within the slideshow. It's very intuitive. Audio, Narration, Titles, Effects and Transitions are all possible and pretty easy to implement.
Videopad, on the other hand is more complex with multiple tracks that interact and is orientated towards video film creation, as its name implies. There are many features more specific to film making and editing. It's a steeper learning curve.
At first sight both programs have a similar interface and both can be used to create slideshows..(still image sequences,) but both can also produce videos. (video clip sequences). Photostage is simpler. Which you choose depends on your requirements. There are forums for both products where advice from other users can be found.
Try them both out before deciding.
Nat