I'm doing something very similar at some festival or other on Friday night in Liverpool. Ideally I'd use my short scale acoustic fretless, as it looks good and as it's got flats on it, it's got a lovely mellow tone that's not too boomy. The singer/songwriter has requested frets though (not sure if he's had a bad experience in the past), so it'll be the Jazz bass. Probably with flats.
Of course, [i]he[/i] gets to play this:
[IMG]http://i1221.photobucket.com/albums/dd471/paul_510/314703_10151889485045332_1495178595_n.jpg[/IMG]