... and yet one more vote for broker's tour first. If you presume that each agent may be working with 1-3 potential buyers who may be a good fit for the subject property, you have now exposed your home to up to 3 times as many people as a public open house can. And you know that the exposure is to buyers who are serious enough to have contacted a Realtor (walked into the lion's den, so to speak).
... at a public open, you're more likely to have casual lookers, curious neighbors and passersby than you are true buyers.
... I'd always opt for the broker's tour first, followed quickly by your first public open, if you're planning on having a public open at all.