Good information there Dr Joe!
I have been pondering this question as well pophead, there is not much information or articles on the web about it unfortunately.
If you have the space, i would go for a DIY wet/dry system.
It will work out cheaper (if you DIY), and be more efficient.
As Dr Joe said, pool/pond filters are for really big tanks/ponds really.
Also, such filters are no where near as effective with fish that have to have perfect water quality. Instead they suit fish like piranhas better, which consequently need more mechanical filtration due to their diet.
What's your stocking intentions? And instead of 300+ gallons, what rough gallon estimate are you looking at?