You just can't compare Murali with Prasanna. But what we have heard from many ex-players his was one of the best off spinners of all time. In the presence of Bedi,Venkat,Chandra in the same team he has an average of 30 and 149 wickets, it is a great achivement and he is definitely better than Murali for below reasons:
1)No other spinner for competition in same team. 2)Clean Action and never suspected. 3)Respect from opposition. 4)Many ex-players like Ian has always told good things about Prasanna.