There is a bit of ground I work Teal on for rabbits, it takes us 20 minutes to hunt mostly bramble cover, on a good day Teal will flush 3 or 4 rabbits most days it one or none, if I hunted the same ground with my old springer Drake it would take 90 minutes or more, that dog ripped up every bit of cover he came across but I am sure he would not produce any more rabbits than Teal does. If you are a single gun working an area that is not thick with game and you don't have a GSP then I would want a cocker Springers waste time hunting non hold cover just for the hell of it but it sure does look good.
In heavy game areas I don't think there is much between them and as for plodders have seen plodders in both breeds, its rare from working lines but it does happen. I do think most plodders I have seen are made rather than born, coming down too hard on a young dog for steadiness at a young age can produce plodders