While Charlotte Z may "consider" Grand Prince the "better" translation, I have a whole slew of Imperial Documents printed in St Petersburg BY the Imperial Press under the direction of Baron Freederickz in English, including a "Handbook of Imperial Russia" published by the Court in 1896 and intended for use of English speaking diplomats in Russia, which all to a one use only the term "Grand Duke".
I quote the Handbook, printed IN ENGLISH in Petersburg in 1896 by the Court, under the direction of Freedericz:
Then follow, firstly the Emperor's sons, daughters and grandchildren (children of his sons); they enjoy the title of Grand Duke, Grand Duchess, and Imperial Highness. Secondly, the great grandchildren of the Emperor in the male line, and all the senior male descendants of the great grandsons, ie: the eldest sons, etc.; they have the title of Prince and Princess of Imperial Blood and Highness. And at length, thirdly all the remaining members of the Imperial House, consequently the younger sons and daughters of the great grandsons etc., are all entitled to the calling of Princes of the Imperial blood, but with the title of Serenissime
Now you might consider Charlotte Z as more knowledgeable, and she is a great scholar, HOWEVER I personally think the Imperial Court itself ought to be the final say on the matter.
Grand Duke. NOT Grand Prince, was the proper address in English at the Russian Court for a son or grandson of the Emperor.