If you can swing it, take crim pro (if your school splits it into crim pro I, II, etc., I'm talking about investigation/constitutional crim pro). Other crim law classes aren't all that important. But don't ditch mock trial to do so.
I'd pick mock trial--this makes a bullet point on your resume rather than a line in your transcript. Many of the firms I signed up to interview with had "Jounal, moot court, or mock trial preferred" in their criteria. None had "Crim Pro preferred."