Commuting on flat terrain/handle bar For my new job I'll have to commute about 10 - 20km (6 - 12mi) each day (one trip) in flat terrain. Right now, I'm commuting 6km/4mi each day, with an elevation gain of 160m uphill to work.

I'm riding a hybrid bicycle with a gear hub (only one gear lever) and a straight handle bar.

I assume that for my new route, a more aerodynamic position would be nice and I'm pondering about a better handlebar choice -- drop handlebars or tri bars as addition to my straight handlebar? Is it just a matter of personal preference, or are there general pros and cons?

The route will be mostly on a seperate bike lane next to a road with little traffic.
When spending more time on your bike, it's good to be able to vary your position. For that reason alone I would advice a drop bar or butterfly bar. It'll prevent all kind of small complaints about wrists, shoulders and back.

Getting in better shape because of the extra daily exercise, you'll gradually feel more comfortable in a more sportive (aerodynamic) position.