My comments echo several of those above, but I've got one point to add.
I've got a 400 f5.6 prime and a 55-300 (both AF and AE). It's a good combination: the 400 for reach, the zoom for light weight and versatility.
What I miss though is speed, especially considering the 55-300, like most consumer tele zooms, needs to be stopped down to f7.1 or narrower for optimum sharpness. I expect it would be the same with the 100-300.
So a key question, I think, is how often you want to shoot at 300mm in poor light. If the answer is "usually", go for the prime. Otherwise go for the zoom, for its AF, auto-aperture, versatility and light weight. For those occasions when the light is insufficient, try a flash extender, like the cheap and light-weight Rogue Safari:
https://www.pentaxforums.com/forums/22-pentax-camera-field-accessories/256288...-extender.html