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You'll have to get in fairly close to get soft light from a 20" box, so that might be something to consider...
A shoot through umbrella (like a 48") is really fast, cheap, and folds up very small. It'll give essentially the same light as a softbox, with maybe a little less efficiency. If you're using it outdoors, you don't have to worry about back spill. If you're shooting tight, you'll probably like the catch lights from a SB better though.
For a softbox, I really like my Lastolite EZ box...it's really nice, but not cheap. I've heard good things about the Cowboy Studio version as well..