I go with Venom.
As Linkalicious pointed out, Venom has easier access to
anti-Clayface weapons and devices, than Clayface does
to anti-Venom ones.
Also, Venom has fouhgt plenty of other super-humans,
while CLayface usually just fights Batman.
I don't know if Venom needs to breathe or not. If so, then
maybe Clayfae, if he captures him, can drown him in mud?