It really depends on what you mean when you say you're trying to track down a ship. If you're just looking for info on the ship, none of these works will really help. Here's what they contain.
Here's an example of what each work contains. This is the 1914-1919 listing for
SMS Friedrich der Große, MSP No. 49.

The listing is just a short blurb. There's no movement information, though for most ships, that would be available elsewhere for the war period. The individual ship listings take up about 2 and part of 3 volumes of the 7 volumes, The remaining volumes go into more detail on the MSP generally, and into specifics for areas that are out of the norm – mail from the Spee Squadron, mail from interned ships, mail from Scapa Flow, etc. The final volume is an index of sort, with a list by MSP number, ship name, or station.
The pre-1914 Crusemann listing for
SMS Friedrich der Große, MSP No. 49.

The pre-war listings have more location data, though the tables can be a bear to decipher if you're not fluent in the shorthand and abbreviations Crusemann uses. The individual listings like these are about 6 of the 14 volumes. The remaining volumes go into more detail on a variety of topics. Ships assigned to certain stations (Ostasiatische Station, Westafrikanische Station, etc.). Ships present for certain actions (Boxer Rebellion, Philippine Crisis, Herero Rebellion, etc.). Which ship's cancels can be found used in which colonies or post offices abroad. Etc., etc. Very voluminous and dense.
To be honest, I almost never touch Crusemann because I primarily collect MSP from pre-WWI, so my go-to is Pohlmann/Kessing. Pohlmann/Kessing isn't as comprehensive as Crusemann in that is omits certain info on more obscure topics (for example, info on exchange of mail at sea, treatment of official mail, etc.), but for those areas where there is duplication – records of movement, discussions of ships assigned to certain stations, present for certain actions, uses in colonies and post offices abroad – the information is presented in a much more readable, understandable format.

If you have a particular ship in mind that you're looking for info on, just let me know and I'll see what I can dig up.