Have never owned a 7x57 double, doubles are medium bore and better in my eyes.

I do have a lovely single 7x57, and the 7x57 is swell but outperformed by the .270 (which is outperformed by the .270 WSM, which is outperformed by the 7mm STW, and so forth

).
I believe the .270 is where my buck stops in that race, it's fast enough (favourite load right now is a 110gr TTSX at 3400fps), and you are right I am having a bit of a love affair with it lately.. It's been the sleeper in my cabinet a long time, under appreciated. It's boring and it bloody well works: Since 1923. I'll be bored of it again by next season perhaps.