Currently I do not give a decibal rating rating to cases. Why?
Well, the fact of the matter is that measurements such as these need to be made in a sound proof acoustical enclosure of some sort in order for their values to have any serious meaning. I see these db, CFM, and temp numbers that other sites use when reviewing cases and I am here to tell you that they are total bunk. As you will see Monday the equipment required to acheive these measurements is expensive and these sites that are provding these numbers are either doing one of two things, winging it or using the numbers provided by the case manufacturer.
This is the bad news, but the good news is that we should shortly have a thermal test chamber and be able to start including thermal testing in our case reviews. If the case reviews continue to be as popular as they have been then, we might consider investing in acoustical and RF testing enviorments. This stuff is expensive and in order for us to do it we need to continue to see that this is something that the readers want.
Let me now get off my soap box for a moment and now answer your question.
The Sonata and Xpider are pretty close, but the Sonata is a little quieter. Why? Well it is because of the grill setup used by the Xpider, which is in fact a better in take to move air through the case, but is some what louder because you get a little more noise that leaks through from the fans.
It is important to note however that the Xpider with the 92mm fan in the fron and the 120mm fan in the cools far better than the single 120mm that is included with the Sonata. What is in fact exceptional about using the 92mm/120mm setup of the Xpider is that it moves a ton of air through the case and it is very quiet. Why? Simple, the 92mm & 120mm fans spin at lower RPMs which creates less noise.
Always remember that larger fans can move more CFMs with less RPMs, hence they create less noise.
The Xpider is an excellent case for a quiet PC situation, unless you use it with 80mm fans then it gets a little louder than with the 92mm/120mm configuration that I have described here.
Good question!