I bit my tongue at the notion of reporting for the umpteenth time the fact that two of the Webley 700s in the care of hunting friends have suffered the same, broken, integrated striker/tumbler over the past few years.
Yes, I've been told, ad nauseum, that lots of guns have integral strikers, and a broken one is rare. Might be true.
But, I've seen enough for me.
I have also been told that Webley 700s exist with disc set strikers, but, it simply doesn't seem to be in the DNA of what the 700 existed for, and at that price point. I've looked at a bunch, and never seen one set up that way.
I'm not talking devils advocate between the two guns you had listed, and guns from the turn of the last century seem to bring their own problems, due to the age they are now. I think the Manton could easily become a money pit. But, I think the Webley, graced with the disc set strikers, would be my choice also. Trust me, having the tool to remove the discs and a pair of spare strikers fashioned will be MUCH cheaper than having the integral striker/tumbler repaired, and if it ever happens that you break a striker, it won't spoil a hunt, which would be the case with the latter.
Good luck, let us know what you end up doing.
Best,
Ted