Both of these heads are created to fill the "Trout Spey" market. This post is to describe some of the characteristics of the heads and how to pick one to use depending on your fishing situation.
The OPST in the picture is at 250gr at 13.5' (different grain weight head varies in length) and the Rio is at 225gr at 11' (all of their heads are at 11'). I used both of these heads on my 6wt converted fly rod.
Since these heads are compact and heavy, you would want to use the appropriate running line for them to cast properly. I use the OPST 35LB Lazer running with both of them.
Presentation- The OPST provided a smoother presentation with the longer head length.
Loading the rod- The Rio definitely loaded the rod faster with the shorter compact head.
Spey Casting - Casting these shorter heads, compact strokes are necessary. You would want to slow down your cast because pulling the anchor out of the water can happen easily. The OPST performs better with spey casting.
Overhead casting with both hands - Overhead casts are amazing on these two lines. But the Rio does overhead cast better in tighter quarters.
Pick the OPST if - you want a smoother presentation and spey cast more often than overhead cast.
Pick the Rio if - you want to quickly pick up your big heavy fly and launch it with very limited back room.