The other basic difference between the competing systems is in the airborne hardware required. If you are among the many thousands of pilots who have installed a Garmin 530 or 430, there is no change required in your instrument panel. The small GDL 49 satellite receiver can be installed virtually anywhere in the airplane. Add a VFH whip style antenna somewhere on top of the airplane and you're in business. The entire datalink system is controlled by the same buttons and knobs you use to operate the 530/430, and the data is displayed on either, or both, of those systems.
To make the Bendix/King datalink system work, you need to install a receiver and antenna, but you also need to install one of the company's multifunction displays. That means you must find more space in the panel, unless your airplane is equipped with weather radar, which, in such a case, the KMD 850 can replace the radar indicator and perform radar and datalink functions. Bendix/ King has delivered a number of its KMD series multifunction displays for use as moving maps, but the number is only a tiny fraction of the Garmin 530/430 systems installed.
Because of the technology choices the two companies made, the cost of each system is very different. First, there is the receiver, which everybody needs. The Bendix/King KDR 510 datalink receiver lists for $5,546. The Garmin GDL 49 costs $3,500. Advantage to Garmin.
But the cost analysis gets more complicated when you consider the necessary display. If you are among the many thousands of pilots who have installed a Garmin 530 or 430, the only hardware cost to add datalink capability is the price of the GDL 49 receiver plus installation. However, to complete the Bendix/King datalink system you will also need a KMD 550, priced at $7,495, plus installation. The company has announced a smaller, lower cost display, the KMD 250, that will be available sometime later this year at a cost of "under $5,000," but for now, the KMD 550 is the lowest cost display you can buy.
It's impossible to estimate what installation will cost because every airplane's existing equipment is different, and the interfaces can be more or less complicated. In the past, avionics shops would often include installation in the sales price of new equipment, but these systems can be complex to install, so expect to pay extra for installation on top of the cost of the equipment.