gravel or sand substrate?

I've owned a few tropical reef aquariums but never a temperate. are the denitrifying bacteria found in sand beds more prominent in tropical environments or are they just as useful in cold water? I have an idea to build a system with a gravel bed sitting on a grate with the main drain underneath to keep the bed clean. I don't know if this is more or less beneficial than a sand bed when it comes to temperate marine. I really know nothing about this side of the hobby but I'm very interested. Any suggestions or even general pros and cons between the 2 are appreciated.