Paddle blenders, also known as stomachers due to the popular Stomacher® brand, use paddles to squeeze the sample against a solid surface and expel the contents of the sample into a liquid phase inside a bag. These are specialized for microorganism extraction and are very commonly used in various microbiological applications, such as food or environmental testing, but they can also be used in some tissue dissociation applications.
Sample size should be your first consideration. Beyond that, the range of bags offered by the manufacturer is arguably be the most important factor in deciding which paddle blender to purchase. Blender bags can be difficult to handle or store, and the right bags can make sample handling and storage much easier. For instance, some instruments have bags that close at the top to help prevent spills during handling, or bags with strainers to better separate liquid and solid phases and allow easier pipetting of the liquid. The availability of other accessories, such as racks to hold the bags, also helps.
The design and motion of the paddles will have the greatest impact on the effectiveness of the bacterial / cell isolation, although this can be difficult for a prospective buyer to assess.