I'm assuming you're referring to ob and psych as specialties. NHSC scholars are PRIMARY care. See Cali-obs posts for what that means in Ob. In my area, ob pay in CHCs is MUCH less than the going salary in private practice. On the flip side, they also work much fewer hours. I would guess the pay for psych would be comparable, in that you would be paid substantially less.
There might be some hope of getting a better salary if you fulfilled your obligation w/ IHS or the prison system, or in a more remote setting (American Samoa, for ex) where you could theoretically get a position which involved a fair amount of inpatient work. But that is NOT likely. It's also important to realize that there are far fewer ob slots than FP slots, which limits you geographically and makes getting a position more competitive.