I just spent some time last week reviewing med ball workouts in the CFJ, on YouTube and on the Dynamax website (lots of ideas there for exercises).
If I understand correctly:
-A 4#, 6# or 8# ball should be sufficient.
-The only people who use heavy Dynamaxes are CFers for the purpose of wall ball.
-Dynamax founder Jim Cawley is vehemently opposed to CFers who have a tendency of not properly using his product, which I presume to be doing 20# wall balls rather than the throwing exercises he advocates on his website.
I don't really have anything to say about slammers, except that they are probably not as pleasant to use when you're doing stuff involving catching.
+1 for TJ Maxx selling cheap stuff in their (women's) fitness departments