Regarding cocker spaniels.... most pet people keep the dogs trimmed. When I was little our neighbor had a lovely cocker spaniel.
I did a search to find something similar to how she kept him trimmed and this link comes pretty close? Scroll down to the black and tan pic. The thing is our neighbor was very particular about keeping him groomed and I think that was a big turn off for my mom who opted to get a golden instead of a cocker (we kids all wanted cocker spaniels because of the neighbor's dog).
American Cocker Spaniel Suburban & Field Trims - PetGroomer.com Magazine
Would not work with conformation where the long coats are a huge deal, but I don't understand why it would or should be an issue with field work.
Regarding English Cocker Spaniels and American - big difference is the coat texture that I can tell???? Maybe it is the individual dogs that I've met, but the ECS's have coats more like setters. I prefer the coats and heads of American cockers.
Where I am - have seen both english cocker spaniels and American cocker spaniels in higher level obedience and tracking. The trainer I know who does tracking with her dogs keeps them in show coats.
Field spaniels - I see them more in conformation. They seem to be excellent junior dogs + not as crazy competitive or difficult to finish as our breed. I have only see one very briefly in obedience and speaking to one of the owners, she grimaced about that dog's trainability for obedience (this was a positive only trainer though so who knows).
@GrandmaToGoldens - brittanies come the very closest to our breed in my mind though still quieter and just slightly reserved? One of the most active AKC obedience judges in our area used to own and work with her show brittanies and these dogs were fantastic obedience dogs (OTCH and CH dogs). Field brittanies though are a lot more dog than these though.