רכוב is superior for he is grasping it - רכוב עדיף דתפיס בה
Summary:
תפיס בה means holding the reins. תפיס בה also includes that it moves a little because of him; as opposed to מנהיג where it moves a great deal.
Or perhaps leading is superior, etc. - או דלמא מנהיג עדיף כולי
Summary:
A מוחזק is one who is holding it in a manner that he could acquire it when he is holding it in this manner alone, even though in this present state (when it is coupled with another entity) it will not be קונה.
Riding alone is not קונה, etc. - רכוב לחודיה לא קני
Summary:
רב יהודה was uncertain whether שמאול was discussing רכוב ומנהיג לחודיה, or whether שמואל was discussing רכוב במקום מנהיג and רב יהודה was [initially] uncertain which one is קונה.