[Bare bottom tanks can look quite amazing. The bottom glass gets covered in coraline algae, a mat of GSP covering it. It certainly keeps nutrients to a low, with good flow you will get small deposits of waste easily removed with a small water cahnge, or they will remain suspended and be removed via filter/skimmer.
Both sand and crushed coral will have nitrifing bacterias, the porusness of the crushed coral is pretty good for bacteria. The problem is crushed coral and other larger grained substrates are monster detritus traps.
A sand bed on the other hand does not allow detritus to penetrate as easily, more likley to be swept back into the water column.
Crushed coral needs to be vacuumed every few weeks, a shallow sand bed needs to be kept well stirred and have filters to allow for good nutrient export.
IMO anyways...I've seen nice tanks kept with DSB, SSB, CC, BB...
If you do go bare bottom I would recommend painting the outside of the bottom glass black or white or cover it like Rick suggested.]