R' Yehuda says that when one is faced with a situation where they can either invest in their own learning or their sons learning but not both, it depends. If all is equal then the learning of the father comes before the learning of the son (just as by pidyon we say that mitzvah d'gufei is more important), but if the son will excel more than the father then the father should forfeit his own learning to enable his son to learn. This needs to be understood better. Would the same halacha apply if I have a friend who will excel in learning more than I will, that I should invest my money in his learning rather than investing in my own? Obviously not, the gemara limits this concept to a son. Clearly, one is not supposed to forfeit their own learning for the sake of someone else, even if that person has more potential. Why then should a father forfeit his own learning for the sake of his son?
The Shulchan Aruch Harav (hilchos talmud torah 7) seems to deal with this issue. He writes:
אם בנו הוא נבון ומשכיל מה שילמוד יותר מן האב, הרי לימוד בנו קודם ללימודו כי מאחר שבלימוד בנו גם הוא מקיים מצוה של תורה כמו בלימוד לעצמו, והרי בנו נבון ומשכיל יותר בלימודו ע"כ
This halacha is limited to a father/son because the very same mitzvah that is mechayev the father in talmud torah, requires the father to teach his son. Therefore, whether the father learns himself or teaches his son he is fulfilling a mitzvah of limud hatorah. It is the very same mitzvah of limud ha'torah "ושננתם לבניך". Therefore, the gemara requires the father to be as effective with this mitzvah as possible. Why? Because the mitzvah of limud includes not just the learning but the mastering of Torah. Just as one would be obligated to use a method of learning that would yield maximum success rather than a method which will not be so effective, so too one is obligated to choose his son rather than himself if that will achieve maximums success. But, this only applies to a son where the father's investment is the same mitzvah of limud hatorah as the father, it would not apply to supporting someone else at the expense of one's own learning.