All that being said, JB's X-Men issues are absolutely crucial to the structure and history of that team in a way that his FF just aren't. If there had never been a JB FF we wouldn't have the Fall of Galactus, Frankie Raye as Nova, Terror in a Tiny Town, the She-Hulk on the team, but we would have Stan & Jack to build upon in the same way JB built his stories upon that bedrock.

What are the X-Men without the Savage Land series, Dark Phoenix, and Days of Future Past? Even today, those form an underpinning to the team so very different from Stan & Jack's original conception that there really wouldn't be much else to work with lacking those. 

Using that gauge, I'd go with the X-Men over the FF, but it's a narrow margin. Also, my enjoyment of the FF title tailed off considerably towards the end of JB's run whereas the more abrupt departure from the X-Men never left that aftertaste, ending with a spectacularly entertaining issue with Kitty living up to her innate Sigourney Weaver-ishness early in life. 

Once JB left the X-Men, the age of the TPB reprint came into being and the fan press was rolling at an all-time high. Article after article piled upon retrospective after retrospective, selling the readers on the quality and impact of that era. New readers could get hooked on the Phoenix Saga at any point and the Phoenix Special Edition kept the focus of the fans and fan press on the Byrne/Claremont era. While JB was no longer on the book, he was still being very much associated with it and those issues were receiving praise and attention at a substantial rate. 

The later success the X-Men enjoyed under Cockrum and Paul Smith were tied in large part to the wealth of attention that had been and was still being directed at the Phoenix Saga and Days of Future Past. That Claremont himself continued to revisit these, time and time again, was telling as well. The growth of X-Men Fandom was reciprocal with the attention paid the JB run, which was never far from the fan's roving eye or dollar. 

Readers clearly liked Claremont's approach, but they could not and were not encouraged to forget those seminal issues done with JB throughout. Those later runs did not succeed solely upon their own merit, unkind though that may be to say, and their success was due in large part to the entire phenomenon, of which the JB run was a crucial and very present part.