The 9400m is worlds better than any of the Intel Integrated you may have used.
A few points
1. The color will be no different with the 9600, that's mostly dependent on the screen.
2. Both will be fine for daily tasks, remember, the 9400m has x264 video decoding support in hardware.
3. OS X will not feel more "fluid", nor will the GUI effects be. The OS is designed to work on machines with GMA 950 and X3100, which are worlds less powerful. If it runs smoothly on them, which it does, it will run fine on the 9400m, to the point where there's no way it would see improvements on the 9600.
4. The 9600 runs warmer and uses more power. There's no reason to use it unless you are: Gaming, doing video, doing something that requires GPU power. Daily tasks will see next to no improvement.