Wet/dry filters can provide great biological filtration and so are especially good with larger tank systems. With large amounts of bio media and the aeration, the wet/dry filter houses massive amounts of beneficial bacteria. They are very simple in design, and a mechanical filtration component can be added to a wet/dry filter. Check out the DIY section. Canister filters are great at mechanical filtration, and bio media can be added to them for bio filtration. A canister filter doesn't have the capacity for the amount of biological filtration that a wet/dry filter could provide. A canister filter, with the right media can provide better mechanical filtration than the wet/dry.
For your 300 gallon tank, you should consider a large wet/dry filter coupled with a large canister or two for mechanical filtration. When we were setting up our 300 gallon tank, we were newbies, and couldn't grasp the concept of a wet/dry filter, so we bought a ProClear 400 (400 gallon capacity) with a CA-6000 pump (1560 gph) I was pretty disappointed in how very simple it is (just like the pictures). We will never buy another one, because they are too easy to make. I know now, that we could easily make a bigger better one. All that aside, the ProClear has provided us with great biological filtration since we fishless cycled the tank. We have extreme mechanical filtration using two Rainbow spa filters run with a Hammerhead pump (5800 gph). While we had excellent water parameters with these two systems, we added an FX-5 (because I had ordered it before the tank was set up) and a 40 Watt UV sterilizer. I think redundant filtration is good, and the FX-5 is available to be used on another tank, should the need arise.
