Most bike tourists in the U.S.A. seem to prefer drop bars. Advantages include multiple hand positions for long days in the saddle and at least one low position to get the rider out of the wind (panniers just have to hang out in the wind, alas!).
From what I've read, many European and "adventure" tourists (trans-Asia, trans-Africa riders, for instance) prefer so-called straight bars (though they're not really straight, as noted above). Advantages are said to include better leverage for wrestling the loaded bike over rough, rocky or sandy roads, and better visibility for heavy traffic.
A few notes. First, I've learned to check traffic from drop bars, so that's not a problem for me. Second, many Americans spend much more time outside cities than inside them while touring. Third, there's not a lot of places in Europe where the only road from town A to town B goes over 100 miles without passing another town.
What kind of road riding are you planning to do?