Yes, I'm aware of that. But my philosophy is to always try to conserve keystrokes wherever possible. Premature optimization is the root of all evil and all that. :) For the same reason I prefer to use dict.items() over dict.iteritems() and range() over xrange() and so on.
This is a simple enough case that I'm not sure it matters, but I think the sum version is simpler to read. Generator expressions are quite powerful if you want to write Python which is more functional-flavored.
