> The first form requires extra work to escape spaces

The first form is generally used for symbol-type strings (word lists, essentially, hence "w"). It is less syntactically noisy, shorter and requires less waffling around with non-word keys.

That's especially flagrant if you don't ignore half the actual array,

    %w(foo bar baz)

really results in

    ["foo", "bar", "baz"]

(the quotes are rather important, because they're noisy, repetitive and annoying to type).

It's one of the few things I regularly wish were in more languages.