In most siddurim I've seen, the word "ונחמתא" in kaddish is written "וְנֶחֱמָתָא," with a סגול ("eh" sound) under the ח.
However, someone recently told me that a certain (very trustworthy) rabbi believes that the word should be pronounced with a קמץ (an "uh" sound) under the ח.
Is either nikkud more correct? If so, why?