So far my view is concerned, I would say that Intl law would be a better and prudent choice especially if a candidate has opted/is going to opt I.R. If this is not the case, Criminology can be opted. But, one must keep in mind that Criminology is a a new subject for CSS aspirants and there is dearth of material also in the form of books through which one may cover it within required time. On the other hand, Intl Law has no such issues listed above.

__________________
"But screw your courage to the sticking place,
And we'll not fail." _Shakespeare, 'Macbeth')