I believe "Missing Link" is just KMC's marketing name for their version of a master link. So, there is no real difference.
Most master links are reusable any reasonable number of times and KMC and Wippermann set no limits on the number of reuses.
A few master links, SRAM's 10-speed Powerlock for example, are not reusable according to their maker. You install them once and, if you remove them, you install a new one aftterward. Whether that's necessary depends on who you listen to.
However, even reusable master links are not a lifetime purchase and, at a minimum, should be replaced with a new one if you install a new chain due to wear.