I'm a propellant chemist, so I'll comment here only about the two very different propellants. (And there are a lot of good things to be said about the newer Jet-X motors, by Mr Roy Lever of UK!)
But turning to the propellant pellets, the original Jetex "fuel" was invented and produced by Imperial Chemical Industries (ICI) of Scotland. Dr James Taylor of ICI led the development team, and among their discoveries were two based upon guanidine nitrate (GN). The first employed an energetic organic nitrate, 2,4-dinitroresorcinol (2,4-DNR). The second used chromates (dichromates, bichromates, etc) with the GN.
[Technically, 2,4-DNR isn't a nitrate, but let's let that slide for simplicity.]
The original Jetex pellets used the 2,4-DNR. When ICI stopped making them, others (including Mr Roy Lever) took up production of the cheaper, easier-to-make GN with chromate mixtures, and they were sold as Jet-X propellant pellets. Commonly they are dark gray, and always the chromate-type pellets burn with very toxic exhaust gases.
Personally, I don't care for chromate-type propellants, for many reasons, starting with the matter of toxicity in handling and use.
Original GN with 2,4-DNR pellets are another matter entirely. It's like comparing fine apples with bad oranges, IMNSHO.
Dr Edward Jones,
Jetex Propulsion Lab