Bigger or smaller softbox?

I'm looking into getting a softbox for use with off-camera flash - mainly for location portrait work (or just nicer snaps ). Lastolite offers a seemingly good setup with the Ezybox Hotshoe Kit - which comes in two sizes: 38x38cm and 60x60cm (18" and 24"). Difference in price is negligible, but the larger one is listed as a 5 day special order while the small one is in stock at the biggest dealer in my city. This makes me unsure - why is demand so low for the 60cm/24"? I thought bigger was better for softer light? Generally speaking, would you recommend going for a larger or smaller softbox?

Thanks!

I have both and I generally find myself reaching for the larger one for portraits. Its more flexible in terms of the area that it will cover.

Thanks for the advice Patrick - I'll go for the 60x60 then.

Hope you don't find this too forwards of me, but I had a look at your blog, and would like to offer my congratulations with the big event in December. It's a miracle every time.