The most searched comparison in this catalogue, and the honest answer is that they are not really rivals. Two different molecules studied in overlapping models, which is why they so often turn up together.
Structure and origin
BPC-157 is fifteen amino acids, taken from a protein found in human gastric juice. TB-500 is a shorter fragment of thymosin beta-4, one of the most abundant proteins inside cells.
Different sources, different sizes, no structural relationship to each other at all.
What each one is studied for
BPC-157 research centres on angiogenesis, the growth of new blood vessels into tissue. TB-500 research centres on actin and cell migration, which is how cells physically travel to where they are needed.
Put simply: one line of research asks how the supply route gets built, the other asks how fast the builders arrive.
Why they are blended
Because those two questions belong to the same process. A repair model that improves vessel growth without cell recruitment, or the reverse, only tells you half the story, so researchers frequently look at both together. That is what the blended preparation exists for.
Practical differences
Both dissolve readily in bacteriostatic water, both arrive as white lyophilised powder, both store sealed at minus twenty. In handling terms they behave much the same, which makes the blend straightforward to work with.
The main practical difference is on the certificate. TB-500 has the molecular weight question attached to it, so check that figure first to confirm you have the fragment rather than the full protein. BPC-157 has no such ambiguity.
Which to choose
It depends entirely on the model you are running, and for a good number of published designs the answer has been both.