In general there's no such things as Stat board pay better than Ministry. The only true thing is that Stat board are more flexible, in that they need not adhere to IM. Therefore they are able to offer other incentives.

At the end of the day, it depends on which agency you are looking at. Certain stat boards like EDB, IE and MAS tend to pay more than ministries. Certain stat board like A*STAR on the other hand, don't follow ministries, and therefore no mid-year bonus nor end of year bonus. So it depends.