If looking for instant or fairly quick turn on, don't go with the UV Cannon. It uses a mercury vapor lamp I believe and it will have a warm up time. Also, the colder the fixture is, the longer it will take to warm up (something to keep in mind with mercury vapor).
I would go with the UV Panel or the UV Wash. The UV wash is basically a more compact UV Panel, but as you can see packs more punch for the size at 100w vs 160w. The florescents in these are basically instant on (unless they are very cold). They won't be as powerful as the UV Cannon however since a) mercury vapor is a better UV source and b) less wattage obviously. They are also a fairly wide angle as well.
The only thing I should warn you about with the UV Panel and UV Wash is the lamps are fragile and it is common for them to break when being shipped and/or transported. Get a back up lamp or two to be safe and also make sure to check the lamps right out of the box. I have ordered UV Washes that had broken lamps in the styrofoam they ship in right out of the box.