I have used both, and prefer SMS far more than I do ECO.
a new dark substrate worth considering is Fluorite Black
ECO comes with plant nutrients, and helps "cycle" your tank.
SMS absorbs available nutrients and is far lighter weight than ECO.
ECO's heavy density helps it hold a slope better than SMS.
Both substrates are dusty, that settles toward the bottom,
but can make vacuuming either one a cloudy affair.
SMS are dark gray discs with a few burnt red specs 2-3mm
ADA are earthy dark gray balls 3-4mm each (light & nutrient rich).
ECO is Black, and heavy, with grains that vary from .5mm dust
through 6mm grains, with the occasional large 12mm rock.
SMS, ADA, ECO