If the action bar is stronger in one design then that will most likely be the strongest system regardless of whether the barrels are held shut by Jones or Purdey.

Much has been said here about the Jones being able to clamp the barrels tight to the watertable but this is really not the right idea IMO.
It is far more important to try and keep the barrel face tight against the action face and on many sxs doubles, rather than having the barrels hard down on the bar, there is actually a gap or clearence between them.

It has also been said that the Purdey bolt is only capable of holding the barrels down where the Jones can cam or force the barrels downward.
IMO this is not totally correct either, as it is not a difficult matter to set the Purdey bolt to apply downward force and guns set this way will usually have the top lever sitting to the right of center when the gun is fully closed.
The important issue is to utilise the camming pressure of the bolting system to force and/or hold the barrel face against the action face when the gun is fired rather than to hold the barrels tight down onto the action bar.

Regarding how much metal remains or has been removed from the bar of the two systems, IMO a top lever hammer gun with Purdey underbolting, will have almost the same amount of action bar strength as a Jones under lever hammer gun because in reality they both require similar amounts of metal to be removed.