What's the difference between Weldtite Dirtwash citrus de-greaser spray and the WD40 chain de-greaser?
Degreasing chains and other metal components is not unique to bikes. There are way to many degreasing products out there to list. On aircraft, we mostly use varsol, but on my bikes, motorbikes included, I have used dish soap and thoroughly dried the chain then oiled it. Lately I've been washing chains in gas mixed 2 cycle motor oil, hehehe, works good.
Dirtwash Citrus degreaser spray is natural environmentally friendly degreaser that is soluble in water, and reasonably easy to rinse off. 
 WD40 chain degreaser is more of a surfactant with petroleum additives. They say it is environmentally friendly, but it contains butoxyethoxy, isopropano, propane, n-butane, and proprietary surfactants.