As someone who has actually used cannisters and sumps (with trickle tower wet/dry systems) I have to agree with Tolak.
On the smaller tanks it can be a pain to even set up a cannister, let alone a sump (though that doesn' stop me having every single tank now with a sump). On larger tanks I would not feel happy without my big towers.
I used to run cannisters on many tanks but gradually changed them over to a centralised filtration system for ease of use (I just do one water change and it covers 3 tanks now).
The plus side to sumps is the ease of cleaning (though I find I have to rinse the filter floss at least once a week or it starts to get real nasty) and the massive bacteria colony they can support.
The down side is their expense and difficulty to set up (I have flooded more than one floor doing this). Also, remember that a wet dry has most or all the media above the water line, meaning a relatively short powercut can decimate your colony as it dries out.
I myself would never touch an overflow box with a barge pole. While it is very unlikely one will fail (if designed well) I prefer to plan for the worst of situations, and as such drill all my tanks. I wouldn't be able to sleep with an overflow box.
In summary, Wet/Dry is great for larger tanks (especially in SW) and can be applied to smaller set ups, but is definately not a panacea.