It doesn't have to be an either/or proposition. The best of both worlds is to have a sump and also canister filtration. You get the increased dilution factor of the sump, along with superb biological filtration (if it is a wet/dry), and potentially increased mechanical filtration/water circulation/aeration with a canister.
On our 300 gallon tank, we've taken it a third step and have extreme mechanical filtration utilizing two spa filters, great biological filtration with a wet/dry filter, and redundant filtration with an FX-5 canister. We have had great water parameters for the last four years. The only issue I have with wet/dry filtration is submerging the biomedia during a power outage to prevent die-off of the beneficial bacteria. (right now, we just open a valve and add more water to the main tank which adds water to the wet/dry), but this is so problematic for me, that on the new system, we are not going to use wet/dry filtration. We are going to rely on two Ultima 4000s for primary biological filtration. One of the Ultimas will run off a 330 gallon holding sump. The sump will also have a Clarity protein skimmer, and the heating, and UV sterilizer will all be connected to lines from the sump. I'll probably throw in some submerged biomedia into the sump, but we really wont' need it. I do like the holding tank/sump idea. BTW, we'll resolve the power outage worries with a generator that kicks on when the power is out. (right now, we do have a generator on the premises that we can use in the event of a long power outage)
IMO, sumps are almost a necessity for really large tanks.