In the article "Deep Impact: Our first look inside a comet" (Australian Sky & Telescope, June 2005, page 24), three scenarios are presented for the nature of Comet Tempel 1, and how it will react to the impact.
http://www.austskyandtel.com.au/back_issue.htm
1) If the nucleus material is relatively weak, and the cratering process is governed mainly by the nucleus' gravity, the majority of the ejected material from the impact will fall back onto the surface, forming a large crater.
2) If the nucleus material is rigid, there will be a smaller ejecta blanket on the surface, and a smaller crater.
3) If the nucleus material is very porous that it gets compressed and absorbs most of the impact's energy (think of a sponge), a much smaller crater is expected, and there would be little material ejected.
We're flying in the dark here (that's why Monday's experiment is so interesting). What do you think will happen?