Many people are familiar with the standard vacuums such as upright or canister, but the utility of those is largely determined by its performance, i.e. the suction. Whether bag or bagless, it usually is a matter of preference, each carrying a long list of pros and cons including affordability, maneuverability, allergies, accessories, and pets.
Originally, all vacuums came with cloth bags that required frequent washing as once the bag become full it lost filtration and suction. While bags still reduce suction when full, manufacturers became aware of the inherent inconvenience of frequent washing and created disposable bags.
While the ability to just throw the bag away is a lot more convenient, it can become costly depending on how much area you have to cover. And, since you cannot see inside the bag, you have no way of telling how full it is, although some machines have a ‘bag full’ light. Also, many bagged devices come with their own filter, which is best for those who suffer from allergies.
Bagless, on the other hand, frees one from having to frequently buy and dispose of bags. So it is better economically and environmentally. However, they usually do not come with built in filters, therefore, you either have to frequently buy them or wash those filters.
Either way, it does not filter to the level of a built in one and the vacuum unit itself has many small openings through which particles can slip, thus, it is not recommended for those with allergies, unless one buys what is referred to as a sealed system. These are harder to find but manufacturers are making more and more due to the increase of allergy and asthma suffers.
Furthermore, it can be a hassle to empty and all those dirt and dust particles can end up back in the air and on the floors.
Finally, the attachments the unit offers should match the type of surfaces your home has and should be able to handle the amount of traffic in that area. For example, if you have hardwood floors, you don’t want something that will just move the dirt around.
If you have children, frequent spills, outdoors indoors, pets, pet hair/dander, allergies or asthma, a second floor, then these things will determine the amount of suction, i.e. level of performance and filtration, that you will need and the overall size, shape, and weight of the unit.
As you can see, the difference between bag and bagless vacuums is negligible. It’s the power that you should focus on.