Word embeddings (e.g., word2vec) have been applied successfully to eCommerce products through prod2vec. Inspired by the recent performance improvements on several NLP tasks brought by contextualized embeddings, we propose to transfer BERT-like architectures to eCommerce: our model -- ProdBERT -- is trained to generate representations of products through masked session modeling. Through extensive experiments over multiple shops, different tasks, and a range of design choices, we systematically compare the accuracy of ProdBERT and prod2vec embeddings: while ProdBERT is found to be superior to traditional methods in several scenarios, we highlight the importance of resources and hyperparameters in the best performing models. Finally, we conclude by providing guidelines for training embeddings under a variety of computational and data constraints.