They both are seeking wisdom and spiritual growth, but for very different reasons. Frankl has to find some kind of order and reason in his experience, or he will either go mad or die. Thoreau's spiritual quest is one of peace and harmony, while Frankl's is one of duress and oppression. He writes, "What matters, therefore, is not the meaning of life in general, but rather the specific meaning of a person's life at a given moment" (Frankl 171). At that given moment in time, Frankl's life did not mean anything to anyone but himself, and he used this experience to develop his own philosophy on life and wisdom, just as Thoreau used his experience to develop his own philosophy. The two men had the same goals, but reached them very differently due to their circumstances. It is difficult to judge who has the best approach, because they both did not choose their circumstances, Frankl had his chosen for him. Of course, Thoreau's approach was much more peaceful, nurturing, and painless, and so it would be easy to say he had the best approach...

However, Frankl's situation was much more demanding, mentally and physically, and so, he may have actually gotten more out of it as a result. He notes, "It did not really matter what we expected from life, but rather what life expected from us. We needed to stop asking about the meaning of life, and instead to think of ourselves as those who were being questioned by life - daily and hourly" (Frankl 122). Indeed, he may have the best approach because he was forced to dig even more deeply than Thoreau for real answers and wisdom. Thoreau could leave his pond whenever he chose, and Frankl could not, so his wisdom may come from a deeper core than Thoreau's, because Thoreau did not have to face the adversity Frankl faced, and so his isolation was much easier and far less stressful.
References

Frankl, Viktor E., Man's Search for Meaning. New York: Washington Square Press, Simon and Schuster, 1963.

Thoreau, Henry David. Walden and Other Writings. Ed. Brooks Atkinson. New York:…