I'll add yet another definition I know of for laterite:  a soil formed in 
tropical/sub-tropical environments where just about all the original rock 
has been eroded, dissolved, weathered, leached away,  leaving only those 
soil components that are really insoluble - gibbsite (Al(OH)3), goethite
(FeO(OH)), hematite(Fe2O3), and kaolinite (Al2Si2O5(OH)4).  The kaolinite 
is a clay, but one with fewer sites (than other clays) available for 
adsorption of iron, magnesium or other nutrients.  the iron components 
(goethite and hematite) are made of the oxidized form of iron.  This type of 
iron is not very useful to plants for a couple reasons: 1) plant biochemistry 
requires Fe2+, not Fe3+, as the nutrient iron, 2)  oxidized iron (Fe3+) 
is very, very insoluble.  It will not dissolve. 
The benefit of laterite is likely to be that the OH's of the gibbsite, 
goethite, and kaolinite do provide some sites for nutrient absorption, 
and, under reducing (low to no oxygen) conditions which are likely to 
occur in some spots in the substrate, the Fe3+ can be reduced to Fe2+, 
which then can be used by the plants.  
As for the pros/cons of laterite (or other clay-like substrate) versus 
EDTA - with laterite, nutrients will be concentrated around the roots, 
not dispersed throughout the water and substrate.  This may or may not be 
important, depending on your plants and fish.  EDTA is 4 acetic acid 
(vinegar) chains attached to 2 nitrogens.  Any organic material put into 
a tank is temporary - it will be eaten by some bacteria eventually.  Once 
eaten, EDTA can no longer prevent nutrient elements from oxidizing.  And 
one waste product from the bacteria eating the EDTA is the nitrogen it 
contains.  However, chelating agents occur naturally, especially in 
black-water streams.  These too get eaten by bacteria ... and those 
bacteria have waste products too.