Most reactors are fluidised so that the media gets more contact with the fluid passing through it. There are pros and con's to fluidised reactors however. My thoughts.
1) Fluidised reactors work by the water flow through the reactor keeping the media in suspense. By doing this the media particles have a longer, more intense contact time with the water. Therefore having a better ability to extract whatever the type of impurity the media is meant for.
2) Fluidised reactors also, due to the way they keep the media particles suspended, simmering like, tend to grind the media particles. Some very fine.
3) media in a bag within a canister filter or trickle filter does not get the desired contact time and is very inefficient in that a lot of media goes unused.
In short. if you use a reactor it is very important to always, and regulaly clean the top sponge/strainer as it clogs very easily due to the grinding. Also the media has to be changed regularly as it becomes extremely fine and will find it's way to your tank. If you can afford, use a reactor with a strict maintenance program. If you decide on the media in a bag option, use a large bag with minimum media. What i am saying is rather have a bag with media in that looks like a pan cake than one that looks like a dumpling. You must change media regularly though.