OK, I tested both fans:

The Xilence instead or having 1400rpm was actually running at ~1245 and the air pressure it generated did not work well with the Ultra. (I am returning it)

The AcoustiFan even though rated at 12cfm less than the Xilence it runs at ~1430 and in fact performs better with the Ultra.

Since I am returning the Xilence I will be replacing it with a Scythe S-FLEX 120mm which is rated for 63.7 cfm at 1600rpm. If it runs close to 1600rpm it should produce the best performance (for 28db!)

Ultra will work best with higher CFM fans, especial the eXtreme which features more heat pipes but same surface area

Well, the Xilence was supposed to be high cfb (68) but it never delivered.

By the way when is the next Fan roundup?

will try for September, as I'm packed until then:) got quite a few nice ones lined up already