I would set it up so you've got 2 speakers each on opposing sides (4 total), each allocated 5 minutes to present their arguments for/against. You can then break it into a panel session, and ask for audience participation.
Of course the lowest tech option of all is to just ask people to raise their hands to contribute to the discussion, with maybe a show of hands at the end to get an overall impression for which side won.
Otherwise an affordable option I like is Sli.do, which will let you do polls and have audience members submit questions via the browser-based app.
Overall I think the debate format is a good one to inject a bit more energy and interactivity into a session.