From my own experience… like most others not actually owning the real thing…
waves version is darker than uad mk1 version, less tweakable, brings mid range out more… quite nice on snares!
uad mk1 definitely brighter and more ‘modern’ and less coloured… incredible on acoustic guitar
i think the waves version is closer to the uad mk2 fairchild than the mk1 although the mk2 is more coloured/characterful/tweakable and sounds more non linear to my ears… waves version is worth having though, particularly if it’s on sale…
as mentioned TDR Comp is very nice too… and FREE!!! … nothing like the fairchild though…