A very interesting question and thank you for asking. Answering obviously will be subjective and much less objective since as dashingscorpio points out the whole is rather large. i must approach this firstly with addressing 'lifelong friend'. How does one define a lifelong friend. Do we determine when life began and then define life with its many phases of life anew and renewed? A puzzle it may seem as to when one decides life may have begun anew contrast life renewed.
Frankly, the oldest friend I have that I have interactions with today I did not meet until 1/3 of my life had passed. I moved my early life as a child, therefore the only friends since childhood I have are my brothers and sisters. The oldest 'best' friend I have has passed. The next oldest 'best' friend I have not been in touch with for say 18 years. That brings the next 'best' friend, of which I am in contact with today and we are at an impasse in life together. Then another best friend I stay current with was also my employer. Today my best friend is my 'Dearest Best Friend' and we live on different continents. We have not physically met.
At this juncture we may address true love. That again will most likely be very subjective and vary as many times as suggested above. For myself I lean toward the understanding of 'true' love using the guidelines offered by C.S. Lewis with his written work 'The Four Loves'. A treatise based on the Biblical foundations, though just may be more universal with a bit and a byte of pondering with other writings concerning 'Love'.
However, the question asked is "which is harder to find?" I am no expert by any venue, criteria, or declare any credibility with scholastic or application of such as a study, therefore offer a perspective that to find one must first seek. We may then ask has what is sought been made apparent of oneself? Perplexing could be said if any length of time is offered simply pondering.
I have a tendency to lean toward there are connections with 'true love' and 'true friend'. I think there is much more power and less force when they are inclusive rather than exclusive . . . do you not wonder, ponder, and/or think?
Well . . . not sure if any of that makes sense at all. I share this little jaunt of pondering is a blessing for myself. One of least has discovered something, even though that may simply be just 'me'. Thank you Nathan for asking this question! You have helped me today :-)