One way to compare proofs is to consider whether they belong to the same "level" or not. Consider by analogy whether a particular Turing machine halts. You can look at the sequence of configurations of the Turing machine at each step. Since the evolution of the machine's configuration is deterministic, any configuration along a "halting path" ends up in the same final configuration (i.e., the first configuration in a halting state).
But that's too difficult in some cases. Most of the Goodstein sequences reach extraordinarily high values before coming back down. How can we prove they all eventually reach 0? Even at small values of n, the sequence length of G(n) requires something on the order of the Ackermann function to specify bounds. We can't inspect these sequences directly to prove whether they reach 0. Instead we create a "parallel" sequence to a Goodstein sequence. Then we prove there exists an algorithm that maps from each item in the parallel sequence to an item in the Goodstein sequence such that both sequences are well-ordered and decreasing. If the parallel sequence reaches 0, then so does the Goodstein sequence. You could think of this as one Turing machine computing the configurations of another Turing machine or perhaps one branch of a tree "cross-predicting" the items along another branch. You aren't just following the branch to its end. In this sense, the proof occurs at a higher "level".
This concept is known as ordinal analysis and one can consider the proof-theoretic ordinal of any theory T. If T_1 and T_2 both prove a specific theorem and have the same proof-theoretic ordinal, you could consider the two proofs to occur on the same "level". Interestingly, Peano Arithmetic can prove that any specific Goodstein sequence reaches 0 but not that all Goodstein sequences reach 0—this requires a more powerful formal system. So if you prove a specific sequence reaches 0 using the more powerful system, I would say that's a fundamentally different proof.