It's "horses for courses"! Depends totally on the handling quality of the gun and whether it suits the type of shooting envisaged. Accordingly, it is difficult to generalise. Probably, the ammunition that is to be used will have as much bearing on deciding on an action type. For example, try finding an English s/s that you can shoot steel through, or something that will handle 40 gram loads for Geese, Foxes, etc.

IMO, very few modern U/O's have the handling qualities of a best London s/s game gun, particularly where the shooting is fast and instinctive. Sure, you could go down to a 20 bore U/O, or look around for one of those great little Browning Game guns for Rabbit or Quail shooting, but, all things being equal, U/O's are usually built heavier tha s/s's.

The exceptions to this are English U/O's like Boss's, or Woodward's. Can't vouch for Purdey's having never handled one.

For Clay targets, where shooting is more deliberate, the U/O rules supreme, but again, the gun's handling qualities are are of equal importance to the sighting plane.Many Sporting Clays shooters are now using 32" U/O's weighing over 8 pounds - try using that on Rabbits bolting through tussocks.

Finally, the shooters individual style has some bearing - depends wither you shoot maintained lead, swing through, snap, or some other system aimed at developing lead.

To illustrate - I use a 32" U/O for Sporting Clays and Ducks, 21/2" English U/O's for Pheasants, 20 bore English s/s's for Rabbits, and 28 bore English s/s's for Quail over a pointing dog. As I say, "horses for courses"!