I have a KX and a K1000.....in terms of build quality and feel there is little between them BUT the KX has better metering with silicon photocells rather than older CdS cells. The KX has a full information viewfinder with match needle metering versus the K1000s single meter needle. I cant swear to it but the KX viewfinder seems cleaner and sharper than the K1000 in my possession.
The KX also has depth of field preview and mirror lock up. Some people think its the best manual body Pentax ever made. MX users will disagree
The KX I have is near mint, the K1000 is an early MIJ version and also near mint and in terms of feel, wind on, shutter release action, overall operation, fit and finish theres not much to choose between them but if I could keep only one it would be the KX, its just more refined and its easier to live with and its match needle meter is really nice.
It seems weird to me that people will pay more for a K1000 which is potentially a pig in a poke deal than they will for a KX which is a thoroughbred but then people pay more for Canon AE1s than they will for FTbs and thats the same story, precison made sweetheart versus plastic and electronic horror. Nowt queerer than folk.
I would think of the K1000 as a K series with all the expensive stuff stripped out rather like a base model BMW compared to an M series.
I have read that Pentax used the K series to experiment with different technologies to decide on where to go next in terms of development.