Color is the first thing you will notice. Second thing is grain size. Then there is what they will do for the tank. Both supply iron. Onyx will also buffer the kH of the water.
On a side note I heard that Wasserpest likes do to strange things with hamsters and duct tape. But seriously, if you are not sure about something check on the web. Here is a quote direct from the Seachem web site
Onyx Sand™ is the ideal substrate for any type of aquarium environment; whether you have reef, planted marine, or freshwater planted aquaria, this naturally black sand will perfectly complement and enhance the appearance of your tank. Onyx Sand™ provides not only iron and other minerals but supports optimal KH levels for freshwater planted aquariums. Onyx Sand™ is most effective when used alone as an integral substrate bed, but it may be mixed with other gravels or sands. Gravel modifiers are not necessary. Onyx Sand™ is not chemically coated or treated but does have a slight buffering capacity. Also available as a gravel.
It took me about 30 seconds to get that information. Not that I needed it as I already knew the answer. If people would stick to what they know and not what they "heard" or "think they know" we would not have as many problems as what we have. If you are NOT sure of something look around the web. If you have questions about a product check the manufactures web site. It normally doesn't take long.