Lumela and tabu
I disagree..!
The split came about when dog shows became more popular, and it was profitable for breeders to breed just for the show market, rather than the dogs needing to be 'functional' in order to sell.
For reasons unknown, the show judges began favouring some physical traits, and the show lines bred for these until there was a noticeable difference between the two.
On a basic level, working dogs are bred for the trainability (how biddable they are and in some breeds, how much drive them have), as well as good health (so they cope with the physical demands of working)
Show lines are bred for how they look. They don't breed out the other bits. If a show lab stud dog has great show confirmation, he will be bred regardless of what his drive/trainability is. And sadly until recently, also regardless of what his hips and coefficient were.
But breeding for looks means you are breeding for a wide spread of 'mental' traits, including all the same traits you'd find in a working line
This means there are plenty of show labs, spaniels, goldies, shepherds, collies etc which have the same mental traits as their working cousins (as there has been no effort to breed those traits out, and often why would you) but they physically aren't up to the job. Looking at the top show labs, they'd never be able to jump over a fence properly, or run all day because they are too heavy and short stubby legs.
I do a lot with labs - therapy dog stuff with my and other dogs, and I also do a lot of gundog training.
I get quite a lot of people coming with 'the family labrador' who they would like to do gundog training with. Normally people new to shooting who want a pet which can also come shooting with them.
It is obvious from a glance which lines they come from, but temperament-wise, I don't think there is an obvious difference
I find working dogs more biddable and people-pleasing, and show lines a bit more stubborn/independent, but I've seen ultra-high energy show dogs and mega chilled working dogs (mine is a working line, all FtChs, and she is the most laid back dog I've owned)
If working dogs are higher energy, it is possibly easier to contain/control it, because they are that much easier to train and keep busy - they will play ball, do food puzzles, hunt for toys for hours and hours. A show lab with a high energy but a short attention span is a disaster zone!
With Spaniels, if you are concerned about their energy levels to the point of getting a show one primarily because they might be lower energy, you shouldn't be getting any sort of spaniel
Sorry for the mamouth post... 