Thanks for all the helpfull comments people (takk skal du ha hajduk ;)). My 21" BB was not snug at the beginning and now they are loose enough to skateboard comfortably in (provided the temperature is 5 celcius as it is here in Norway at the moment). So I'll think the 31" is the way to go.
Off course, now I face the dillemma of how much hemming. To cuff or not to cuff, that is the question. But I have to figure out that for myself.