Hard to give a good answer.
Generalising, and therefore being wrong in many cases, my impression is that the earlier ones
- contain more eyewitness accounts
- don't waste so much space on 1866 and 1870/71 ;-)
- are more frank on aspects which are not so positive, whereas those written in the German Reich tend to remain silent on events touching the perceived "honour" of the regiment.
Amongst the many counter-examples to my general impression are Mackensen's excellent histories of the Leib-Husaren.
Anyway, there are not so many regimental histories that we couldn't study them all, and add to our reading the published memoirs of the regiment's members ;-)
The best bibliography of German regimental histories (not only Prussian):
Bibliographie zur Heeres- und Truppengeschichte des Deutschen Reiches und seiner Länder: 1806 - 1933, Band 1
Autor: Eike Mohr
2nd edition, Biblio-Verlag, 2004
ISBN 3764823313, 9783764823313