The only difference: CSI centers around physical evidence gathering & collecting the pieces to solve a puzzle.
Criminal Minds involves more psychology & sociology. We, as viewers, want to know what makes someone tick as a way to prevent crimes from occurring again.
Another reason why I enjoy this showed so much is how the characters put their expertise in play to solve some of the most twisted people around.
As casting is concerned: Patinkin is perfect as Jason Gideon. If you think about it, he's more like a professor citing famous lines from Einstein, Faulkner, etc.
When I first read those lines, it didn't make sense at first, but as you watch each episode , the lines that Gideon cites has more meaning to it.
For example: Einstein: "Imagination is more important than knowledge.Knowledge is limited. Imagination encircles the world." Gideon is saying that the human mind is the deadliest weapon of all, & we are capable of anything & everything of achieving all kinds of possibilities.
In conclusion, Criminal Minds is the kind the of show that anyone can enjoy.