I'm one of the authors of Quaterion and also a contributor of TF Similarity. First of all, Quaterion is based on PyTorch. From a more technical perspective, TF Similarity is currently stronger on the self-supervised methods for images and lacks some more generic losses while Quaterion puts a bigger emphasis on finetuning models from any domain efficiently for practical purposes. For example, Quaterion has a intermediate caching mechanism that accelerates finetuning considerably. Overall, They both do a good job for their own use cases.