I would say that 600 nits would be overkill, especially if you have tints. Im guessing that you would have more trouble with the 600 nit version being too bright as apposed to the 220 being too dim. And think to yourself how much direct sunlight will the lcd get. The place I'm putting my lcd gets very little sunlight because it is fairly low in the console and I also have tints.
Just my opinion.