Let me preface this by saying that I have not watched St. Mary's at all and am going just by what I have read.  It sounds that they have 3 main players in Mills, Simpson and Samham.  I look at the matchup like this:

1: Rochestie vs. Mills, both are good, but it sounds like St. Mary's has the advantage
2: Klay vs. Hughes/Hunter advantage WSU
3: Does it really matter, anyone pretty much has the advantage over WSU here
4: Simpson vs. Forrest, sounds like advantage St. Mary's and will probably need more Casto.
5: Baynes vs. Samham, again both are good, but I would have to say advantage WSU.

They have a deeper rotation as far as the number of guys that get time, though it seems they mainly have 7-8 getting the majority and most of the guys who play seem to be guards.  I guess without any other knowledge I would have to give this a push.  Looks like they have the advantage with guards and the Cougs have the advantage with bigs.  We'll see how the fouls go.

Looking at their roster they have one other big body if their big guy gets into trouble, other than that, most of the playing time goes to other guards.  I think Capers and Nik will help on D.  Looks like Forrest may be outmatched here so Casto should get some good minutes if he can stay out of foul trouble.  I think the biggest challenges are going to be Mills and Simpson, probably Simpson the most.  If they don't call ticky tack calls, Baynes should negate Samham and hopefully get him in foul trouble, if they double, Forrest could be open, but then Forrest might not be the best option for D. 

I think our biggest advantage in this one could be Klay, as long as he does better than he did in the Pac-10 tournament.  It doesn't appear that they have anyone who can match up with him.  Hopefully he gets going early.  Looks like they will either have to put a shorter guard on him or a forward who probably isn't used to guarding out there.