Of the Bach fugues do you consider the C sharp major difficult to memorize, or do you advise the use of the D flat arrangement instead?

Such little differences have never bothered me, and I can therefore hardly answer your question definitely. It has been frequently observed—though never explained—that to many people it comes easier to read music in D flat than in C sharp. Hence, if you prefer the D flat edition it will reduce the difficulty for you. Possibly this more accessible version may aid you optically or visually in your work of memorizing.