The simple difference between a Multi-Edition Collection (ERC1155) and a Single-Edition Collection (ERC721) is that 1155s allow for collections of collections, while 721s allow for singular collections of a single item.
Single-Edition Collection - example
Multi-Edition Collection - example
In 1155 collections, each edition has a unique ID, but the tokens within do not, meaning that multiple people can own the same token ID. On the other hand, 721 collections allow each token within a collection to have its own unique ID. Another key difference is that 1155 collections are stackable items, meaning you can continuously add more to a collection, while 721 collections are non-stackable items. As a result, 1155 collections are better for creating for the masses and are easier to curate.
However, 721 collections have their own advantage in allowing token IDs to be sought after. Each token ID has its own unique value and can be considered more valuable depending on its rarity. This makes 721 collections ideal for membership passes or collections with smaller numbers of items where token IDs can be coveted. Overall, 1155 and 721 collections have unique use cases and advantages, and the choice between them depends on your needs as a creator.