I didn't notice any difference between DX and GL.
I think it's more to create a better 3d experience, used by VR, 3d anaglyphic, head tarcking, but also for non windows machines (as they won't dupport DX9).
I installed RC6, as well and experience the same issue. it just hangs after the message "VPX Full screen exclusive focus hack set".
Turning this off resulsts now in the same issue
@wrd1972 Can you do a test for me ?
Rename in the PinballX folder the file PINemHi.exe to PINemHi.exe_ and redo your test (with DX or GL, but I only checked DX). For me itresulted that I could launch, play and close tables and return to PinballX agan (even with "Full screen exclusive focus hack" enabled)