Narrow iron on interfacing - what would be the most useful width?

If you could buy a narrow, iron on interfacing strip (like a narrow Vilene) for making things like the straps on the top above, what would be the most useful width size? 1cm (3/8in) or 1.3cm (1/2in)?


I would like to have both sizes available, because I could imagine using the 1/2in for narrow bag straps, but maybe the 3/8in one is more elegant for clothes?

Comments please?