Like labrador said, many of top tier scholars (not including those who has left) are AOs. There are second tier scholars who are AOs but far fewer. Stat board scholars simply do not get the same level of exposure or relevant jobscope as compared to PSC scholars. I'm sure you know that ministries formulate policies, stat boards carry them out. There isn't any policy formulation done at stat board level. No one really wants a stat board scholarship, just ask any top scholarship applicant. But many do take it up after being rejected for a top PSC scholarship. And it pays for an expensive 4-year education so they take what they can get. I dare say it's a well known fact that almost all stat board scholars are PSC-SAFOS and PSC-OMS rejectees. (meaning they applied for PSC and stat boards and didn't get PSC)

Why would anyone with brains turn down an OMS for a stat board when the OMS scholar will be the stat board scholar's boss?

All stat boards come under the authority of a ministry. Eg MTI governs STB, EDB. Mindef governs DSTA. MOF governs MAS and so on. The stat boards execute menially the policies formulated by their parent ministry (done by the PSC OMS and SAFOS scholars there). So essentially, a stat board scholar is doing the sai kang with no exposure to policy making.

Stat boards are also totally independent of each other. Eg when a MAS staff wants to move to say, PUB, he resigns from MAS and then reapplies to PUB. There is no fluid movement for people in stat boards.

PSC-OMS scholars are rotated at the highest level of policy making in various ministries and they switch between ministries fluidly. Their scholar friends they befriended in JC and Yale are stuck at the stat board, with no lateral movement.Do note that the description above for staff of ministries ONLY apply to high fliers like OMS scholars. Their regular rank and file don't have. I'm just explaining the set-up of ministries and stat boards to you. The privilege doesn't extend to any tom, dick or harry.

To your second question, the pay between the two isn't even comparable.