I purchased Complete Canal Priests Of Mars after reading the original GDW version. The "Complete" version you can think of as a Director's cut edition. Not only is there more encounters and background, there are many more excellent illustrations and player handouts compared with the original edition.
Whether complete or original, the adventure is well-written with a bit of mystery, and an appointment with fate. The adventure has a plot, which brings up the familiar problem of keeping the characters on track, while still allowing for player agency. The author handles this well I think, with each encounter written to answer for several different outcomes based on player success and choices.
Knowledge of the British Victorian "atmosphere" of Space 1889 is one of my weak areas, and always made me reluctant to try the game. However, Canal Priests Of Mars, in particular the Complete edition, seems almost written to help that particular weakness of mine, chock-full of rich detail of the time period.
I would suggest this as the #2 book you buy after the Space 1889 core book. Excellent.