I have an English cocker spaniel - show line. She is a lovely dog - she's 2 years old now. Very chilled, very happy and very easy to work with. Mild and loves people, including children. She gets kisses and hugs from kids and adults in the neighbourhood - she doesn't care if she knows them or if they are strangers, she just loves it.
I take her out 3-4 times daily, just 10-20 minute walks each time. And then spend around 30 minutes training her every day - fetching, different obedience training and maybe some food search. If we meet any of the dogs from the neighbourhood, they play until we have to go/dogs get tired.
My parents have a field spaniel. I think it's a cocker, but I can't remember right now. She's around 7 years old now, and requires a massive amount of exercise every day. If not, she's almost crawling on the walls. She chilled a bit around 18 months, but still needs 1-2 hours of exercise daily to be chilled around the house.
She's just as mild and lovely.
My parents were dog sitting my dog last summer, and they were surprised how big the difference is.
None of the dogs have ever growled or nipped anyone, and have no problems with possessiveness. They hardly ever bark, although my parents' dog sings along to the church bells.
Both are velcro-dogs though, so if you're not keen on having a little 4-legged shadow following you, don't get a spaniel.
I went for the show dog because of the energy levels. My dad was hunting for many years, so he used the dog as a hunting dog. Now, they don't go hunting, but my dad still trains her every day, goes on the bike with her running next to him and goes to the beach to get her fetching in water.
My very first dog was a cairn terrier, and he was lovely - but so different from a cocker.