1. Key difference 1: Algorithm focus Gensim is a Python library that focuses on topic modeling and document similarity, providing algorithms such as Latent Dirichlet Allocation (LDA) and Word2Vec. On the other hand, Transformers is a library primarily focused on Natural Language Processing (NLP) tasks, providing state-of-the-art models for tasks like text classification, translation, and question answering.

2. Key difference 2: Model architecture Gensim typically uses traditional Bag-of-Words or TF-IDF approaches for representing documents, and its algorithms are based on statistical models. Transformers, on the other hand, introduced the concept of attention mechanisms and Transformer architecture. This architecture allows for capturing global dependencies and effectively utilizing contextual information, resulting in better performance on various NLP tasks.

3. Key difference 3: Pretrained models Transformers provides a wide range of pre-trained models, including BERT, GPT, and RoBERTa, which have been trained on large corpora and fine-tuned for specific tasks. These models capture contextual representations of words and sentences, enabling transfer learning and reducing the need for extensive training on specific tasks. Gensim, on the other hand, does not include pre-trained models out of the box. It provides tools for training custom models using different algorithms.

4. Key difference 4: Training data requirements Gensim algorithms typically require a large amount of clean, preprocessed text data for training. This is because they rely on statistical models that learn patterns and distributions from the data. In contrast, Transformers models can leverage large labeled datasets for pre-training and then be fine-tuned on smaller task-specific datasets. This reduces the reliance on specific domain data and allows for effective training with limited data.

5. Key difference 5: Language support Gensim supports a wide range of natural languages and provides tools for building models for different languages. It has features like language detection and tokenization that facilitate working with multilingual text. Transformers also supports multiple languages, but its pre-trained models are mostly focused on English. However, there are efforts to expand the language coverage of Transformers models.

6. Key difference 6: Performance and scalability Due to its focus on specific algorithms, Gensim is generally considered to be lightweight and efficient for topic modeling tasks. It can handle large text corpora and provides fast implementations of algorithms. Transformers, on the other hand, is built on top of deep neural networks and requires more computational resources. It tends to offer state-of-the-art performance in NLP tasks but may require more powerful hardware and longer training times.

In Summary, Gensim and Transformers differ in their algorithm focus, model architecture, availability of pre-trained models, data requirements, language support, and performance/scalability.