Google's Page Speed has some information regarding using efficient CSS selectors. I suggest starting there.
So (very) basically, they recommend to:
Avoid using descendant selectors, especially those that specify redundant ancestors
So your second option is the fastest.
I would opt for the former snippet and not worry about any efficiency whatsoever; unless you have a dozen or so stylesheets CSS won't be a bottleneck. The former snippet is more readable and you know exactly what you're styling, and where it belongs structurally, so when you jump back to make edits you know what to search for.
Then again if you're making global styles like a class to contain floats it would be better to keep it simple.
"Speed optimization" really shouldn't be of any concern with CSS. The difference in any modern browser is marginal, but the difference in usage is not. You should decide when to use "narrow" or "broad" selectors based on what you want to achieve, how far you want styles to cascade, where you want them to be applicable and if you need to override a broader selector.