Interesting. I have been contemplating the same things and watching this with interest. I am a real fan of Eheim and had been considering the 'wizzing past the substrate' issues with the FX5 as well. I am now actually leaning toward the FX5, here is why:
1. Filtration is good but a lot of beneficial bacteria is in the tank, substrate, etc. etc. so the filter is likely to be sufficient for the bioload, regardless - in other words, it isn't really an issues in this comparison.
2. Aerobic bacteria are very beneficial in the nitrogen cycle and with increased flow, there is better oxygen availablity - in lieu of a wet/dry sump, this is potentially the next best thing.
3. I have noticed significant benefits to high flow with respect to plant growth and often with fish health as well - just my observations but I don't want to fix what isn't broken.
Going to buy next week so I need to decide for sure ASAP. :roll: