The most important difference is that the 26-1k uses a mercury vapor lamp, the 26-1ks a metal halide lamp. Both units radiate about the same amount of useful UV radiation, but the metal halide lamp reaches full radiation after turn on in less than half the time of the mercury vapor lamp.
Another consideration is that the 26-1k is the older of the two units, which means there is a good chance it will have received a lot more use than the 26-1ks.
Any body can tell me where I can get Nuarc 26-1KS bulb. The only information I have ison the bulb is: Nuarc X2T and NP4 44/1.