I am in the process of doing this very thing. Replacing my 2 HOBS for 2 medium canisters. I think it's just more of a personal preference really. My HOBs have been working great, but I don't like the water fall that I get because it knocks my fish around if they pass under it and it is noisy. I liked the sound the first year, but now I hate it. I did eleviate the waterfalls for awhile by keeping the water level high enough the filter flow would just ripple across the surface, but that didn't work too good when we had an eartquake 2 weeks ago. Water was slopping out both sides of my tank and I thought fish were going to be carried right out with it!
I also like that canisters have more room for biological filtration as well as being hidden away from view