How electronic controls work (or could work)
There are two key elements in electronic scoring: electronic control which is set at the location in the woods/street and an electronic card (often called chip or dibber) which is carried by runner and is used to identify each runner. Important part is the timing system - it can be done either way. In this comparison, timing system is the "smart" part, and the simple systems which just store data are called "dumb".
Smart control and dumb card: Standard SportIdent uses this arrangement, where controls in woods are smart - they are used to calculate time and to store runner IDs. Cards are stupid and used only for storing time and visited controls. OPORCON is also this kind of system. I presume Joe Lee, Slovenian and Clifford Heaths' system are in this category too. Controls are expensive, chips are cheap so the system is easily deployable to many, but theft of the controls is a problem.
Dumb control and smart card: Emit uses this arrangement. Controls are pretty much stupid, and they just identify to cards runner. Cards have internal timing system and memory to record when each control was visited. The system that was made at my college is in this category. Controls are cheap, chips are expensive: system is expensive per user, but setting a course is easy and cheap. Controls can be even left in the woods or streets on permanent courses.
Dumb control and dumb card: Standard punching system. There is no timing, and only thing organizer can know is which controls are visited. There is no sure way to check the order of visiting, unless you're using a GPS system. Dumb/dumb electronic system can have the order of visiting, but without timing - first versions of OPORCON worked this way. Another way of electronic dumb/dumb would be a system which saves data to a control, but then instant results on the finish are impossible. If control sends card (runners) ID over the air (GSM, Wireless), finish results are possible, but that control can't be called dumb. Controls and cards are cheap.
Smart control and smart card: Wireless controls like new Emit and SportIdent controls need a lot of smarts to work properly. Even if they don't have timing in each element, they can be easily incorporated. Controls and cards are expensive, thus not used widely.
To be honest, systems described here are those relevant to OPORCON - similar home made systems, and IOF approved systems. Not described are Chinese systems and Russian SFR but those are not widely available (although they are cheaper than SportIden and Emit).