America's ship-launched X-47B killer drone prototype took off for the first time from the aircraft carrier USS George H.W. Bush sailing near the Maryland coast on Tuesday morning – the first step in proving that a high-performance Unmanned Aerial Vehicle is compatible with the Navy's fleet of 10 gigantic nuclear-powered flattops.

But that doesn't mean the sailing branch will definitely be purchasing similar jet-powered drones for frontline use. According to Bob Work, until recently the Navy undersecretary and a big supporter of armed UAVs, the sea service must choose between X-47B-style 'bots and a simpler, propeller-driven drone similar to the Air Force's Predator.

The Navy's wager makes could determine the course of U.S. air power for decades.

The X-47B and planned follow-on models offer high speed and the ability to evade enemy radars, but at a cost of potentially tens of millions of dollars apiece. A Predator-esque drone would be slower and non-stealthy, but also far cheaper to build. "You can't afford both; you have to make your bet," says Work, who left the Navy in April to head the Center for a New American Security think tank in Washington, D.C.

There's more at stake than just money. Jet-propelled killer drones are best suited for quick, hard-hitting bombing runs against heavily-defended military targets such as missile sites, airfields and command bunkers. Think robotic air war with North Korea, China or Iran.

By contrast, slow-flying prop-driven drones are better for hours-long missions patiently scanning for low-tech insurgents then hitting them with lightweight missiles – which is exactly what the Air Force's Predators have been doing in Iraq, Afghanistan and other low-intensity battlegrounds for more than a decade now.

In short, the X-47B and related drones are for conflicts just short of World War III. The Predator-style models are for the small wars, chasing terrorists and insurgents in what Work calls the "global manhunt." Choosing one over the other, even for reasons of cost, could determine what kinds of wars the Pentagon is prepared to fight – and by extension the kinds of wars policymakers might be tempted to fight.

Plus, the Air Force is closely watching Navy drone development, hoping to piggyback on the sailing branch's efforts in the event the Navy chooses the jet-powered model. If the Navy decides to go with a X-47B-style drone, the new flying 'bot could quickly proliferate.

Until now the Navy has hedged, investing in both types of drones and delaying the moment when it must choose. The sea service has trimmed and stretched out $800-million worth of work on the X-47B, in development since 2007. And it wasn't until this March that the Navy, in conjunction with the fringe-science Defense Advanced Research Projects Agency, finally launched the Tactically Exploited Reconnaissance Node, or Tern, meant to create a ship-launched drone similar to the Predator at an initial cost of just $9 million.

But much bigger bills are coming, forcing the Navy to place its bet. The Unmanned Carrier-Launched Airborne Surveillance and Strike effort, aimed at producing a frontline follow-on to the X-47B, could cost more than $2 billion over just the next five years – and billions more if and when it enters full-rate production. A typical jet-powered UAV costs up to $15 million to build.

A Tern drone, or something like it, could be comparatively much cheaper. The Predator costs just $4 million apiece, meaning the Navy could probably design and purchase an entire fleet of Predator-class drones for less than a billion dollars – not a huge sum, by military standards, but huge enough to preclude overlapping UAV programs. (Funding for drones is dropping across the Pentagon budget.)

For his part, Work implies the jet-propelled drone is the safer bet, as it's better suited for taking on the most powerful potential U.S. rivals – Russia and China come to mind, although Work did not specifically name them. Moscow and Beijing are both working on new missiles, stealth fighters and even drones of their own. "Everybody is surprised at the pace and broad range of capabilities adversaries all over the world are pursuing," Work says.

Plus, the X-47B and similar drones could in theory perform the surveillance missions that are today assigned to slower-flying Predators, although the robotic jets would be less efficient at such tasks. But the reverse is not necessarily true. Prop-propelled drones probably would not be able to penetrate high-tech enemy defenses like the X-47B could.

The jet-powered drone is likely more flexible, but flexibility might not matter if the Navy decides saving money is the better wager for the future of robotic U.S. air power.