@ Darter is cheaper. Exactly how much cheaper depends who is buying and what kind of deals they can offer (oil instead of cash, investment or aid programs and so on.)
@ Darter is designed to be used on older aircraft (like MiG-21s and similar ancient warplanes found in places like North Korea, Iraq and China.)
@ The Darter doesn't have a mid course target update like the AMRAAM.
@ The Darter is more agile than AMRAAM, and can be used more effectively close (that is, in the traditional dog fight).
@ The Darter is designed to use the Israeli helmet mounted sighting system, allowing a pilot to quickly switch between short heat seekers and longer range Daters.
@ Also unlike AMRAAM, Darter has a lock (on the target) before launch mode.
@ The Darter is designed to be launched almost simultaneously with a heat seeking Python missile. This makes it more difficult for a close in target to evade getting hit.
@ The South African Darter probably won't have the same range as AMRAAM or Derby, or electronics that are as capable.
Nations like Iraq, China and North Korea provided moral, and some material, support to black South Africans during the struggle to overturn white rule. The current black majority government in South Africa still has warm relations with these nations and would be inclined to sell a few hundred Darters to their "old comrades" for past services, and to hell with the United States. The only bright spot to this is that Israel and South Africa do not cooperate in weapons development as much as they used to. As a result, the South African missile's development did not go as smoothly and is probably not quite ready for prime time yet. Moreover, the U.S. Air Force watches these developments closely, trying to develop ways to deal with these new missiles before American pilots encounter them in combat. And the potential for that is there, and South Africa needs the money.
While the United States scrambles to keep high weapons away from hostile nations like Iran, Iraq and North Korea, or potentially hostile ones like China. Russia, France and Israel have been the biggest problems for America, as these countries are inclined to sell to just about anyone. America's long weapons research relationship with Israel has caused additional problems in this area. Israel gets America's most modern weapons (they pay us with money we give them to, well, buy weapons.) Israel also produces it's own high tech weapons. Not surprisingly, Israeli designed and manufactured weapons often are very similar to American ones. These accusations of technology theft are a constant sore point between U.S. manufacturers (and the U.S. government) and Israel. A further complication is Israel's long relationship with South Africa. Back when South Africa was ruled by the white minority, and thus prevented from buying weapons from Western nations, Israeli and South African scientists worked together on a number of military projects. One of those was the Darter long range air-to-air missile. The Darter was a joint project (the Israeli version is called the Derby or Alto). Both the Israeli and South African missiles are based on the Israeli Python, which in turn was developed from the American Sidewinder. The most significant differences between Darter and AMRAAM are;