Both are excellent...just depends on what you want.
Calmini bars are lighter weight and tuck up close to the body for improved clearance and a "tidy" look. The fact that they are closer in also means that there is less leverage on the frame when you rest on them. The drawbacks are that they don't offer much side protection and you can't use them as steps.
Schrockworks are much larger and heavier. They provide a place to step and offer better side protection. There is no question the Schrock bars are top quality and well built. Drawbacks are that they hang lower and if you come down hard on them they will have more leverage on your frame.
My truck is 800 lbs overweight and I was very concerned about the leverage issue. Some people have bent their frame or popped bolts with the wider bars. I went with the Calmini bars and have been very pleased with them. Actually, I have never set down my Calmini bars on anything, but the ones I had before that hung lower and wider were dragging on everything.
I wheeled for a long time with no bars and never hit anything. Since the Xterra is built with the body on top of the frame the body is up pretty high and there's quite a bit of room before anything will hit it. Unless you're doing a bunch of rock crawling or you are not careful, there shouldn't be a problem going without. Just depends on you and what you are doing!