The calorie count for California Rolls depends greatly on the amount of sugar in the mirin, and the amount of avocado put in the role. I've seen California Rolls that ran 40 calories per piece, and I've seen California Rolls that ran 90 calories per piece. Rely on the nutritional information availble where you buy your sushi, and if they don't have nutrtional information available, always use the higher number.
Tuna Rolls, without avocado, generally run about 25-30 calories. I've seen some Tuna Rolls, though, that could very well have been 50 calories per piece, given how big they were.
Sashimi is, of course, the least calorie sushi, and has the lowest Glycemic Load.