The Internet is currently obsessed with Amazon's plan to deliver packages via unmanned "octocopters." Amazon CEO Jeff Bezos believes the technology may be ready within five years, and that they could enable packages to be delivered in as little as 30 minutes.

Aerial drone technology could revolutionize the retail business, threatening not only other online retailers but some brick-and-mortar ones as well. But flying robots probably won't be the only automated delivery technology entering the market in the coming decade. They'll have to compete against self-driving vehicles that drive around on the ground. And ground-based delivery robots may be the winners in the long run.

"The ones on the ground should be the long-term winner because they're going to be more efficient," says Brad Templeton, chair of networks and computing at Singularity University who has been writing about the potential of self-driving vehicles, including delivery robots, for more than five years.

And surprisingly, Templeton argues, ground-based delivery robots will be "able to go to more places." Templeton argues that if you look at satellite pictures, "you'll find that even as a human being, you're not sure how to land on peoples' yards." Obstructions like trees, power lines, tall buildings, and the like can make it tricky to land in some parts of town. High winds or other adverse weather conditions can make drone deliveries even more challenging.

The problem of piloting a self-driving vehicle is comparative simple. And once ground-based vehicles don't need human drivers, they can be made extremely small, simple, and cheap. Templeton envisions "robots the size of a dog" driving around urban streets at modest speeds. The small size, low speed, and lack of human passengers means that safety wouldn't be a big issue. That might allow the technology to be introduced earlier than full-scale self-driving cars.

Flying and driving robots have different strengths and weaknesses. Bezos says you won't be able to have table saws or kayaks delivered by Amazon's drones. But these items could easily be delivered by a self-driving ground vehicle. On the other hand, a ground-based vehicle is never going to get to its destination quite as quickly as a flying one can. So if consumers want their products in a hurry, Amazon Prime Air may be the best option. The of the future might offer customers a choice between getting 60-minute ground delivery for free or 20-minute air delivery for an extra fee.

One challenge for both delivery types is what to do with the packages upon arrival. Here, Templeton argues that the flying machines might have an advantage: they can drop packages in peoples' back yards, where they're less likely to be stolen. They could drop them on the roofs of apartment buildings, which would only be accessible to building residents. Ground vehicles, on the other hand, might be given a code to open customers' garage doors, though that could raise security issues of its own.

But in the long run, Templeton predicts, this problem will go away entirely as delivery companies increasingly deliver packages directly into the hands of recipients. With delivery times measured in minutes rather than days, vehicles could go directly to the customer's physical location. That might mean delivering packages to users while they're at home. It could also mean using the GPS on the customer's phone to visit the customer wherever he happens to be when a package is ordered: at work, the dentist's office, or a child's school. Drones, Templeton says could "get rid of this idea that we deliver on the delivery company's schedule and deliver on your schedule instead."

Most likely, the future of retail will use a mix of ground- and air-based delivery vehicles. Air-based vehicles make for better television. But the lower cost, higher capacity, and greater reliability of ground-based vehicles means they're likely to be the workhorse of 21st Century retail.

Disclosure: Amazon CEO Jeff Bezos owns the Washington Post.