I really don't think the two can be compared, with one being a comedy and the other a drama. I would have to say that unlike a lot of the posters in this thread I believe that Psych has way higher production standards than The Mentalist.
The writing on Psych is also far more intelligent and superior to the mentalist. Psych is not trying to take anything serious, especially the show itself. The attention to detail on Psych is what makes it really shine.
The flash-back scenes paint the background for why father, son and best friend have grown up to be who they are and give much more depth to the characters than anything on the Mentalist, and that includes Jane with his back-story of swiddling people to the death of his family.
The way Psych lets you see the things that Shawn picks up on and then try to figure out how it fits into the mystery is really nice. Jane often just walks into a room and decides without even looking at the surroundings or talking to the people what happened and who did it, like he really is sycic, then the rest of the show is about catching them with some mind trick fake out.
People say Psych is outlandish for what they get away with, but Psych is supposed to be a comedy and be absurd. The mentalist has Jane do things that he would be locked up for period with no chance of being saved by the department. He breaks the law in more serious ways all the time than Shawn and Gus ever do. Like locking the guy in the barn and dumping flammable fluid on him. Most of the arrests made on the Mentalist could be thrown out of any Court with the defendant representing themselves there are so many breaks with procedures and rights violations. Its so over the top in its own way its absurd too, which while fine in a comedy that is trying to be absurd is not fine for a crime drama.
The attention to detail like changing the opening and closing theme music to fit the episodes, the much more intelligent dialog, and the fact that all the characters are just plane likable on Psych makes it a much more enjoyable series to watch. I've watched both series from the beginning and think Psych is one of the best series I have ever followed (age 44 so I have followed a lot of series over the years) and the first series that I have ever purchased every single episode for (Amazon VOD / Roku player), where as the Mentalist is getting old already and Janes attitude and character when dealing with even his fellow team members is beginning to seem like House without the redeeming parts...that sums it up nicely...House has no redeeming features to his personality and he is still more like-able than Jane.