a) 彼らがお前を否定するなら if they deny you
b) 彼らがお前を否定すると言うのなら if they say that they deny you
a) お前がどうしてもしたいなら、止めはしない。 If you really want to do it, I won't stop you.
b) お前がどうしてもしたいというのなら、止めはしない。If you insist to do it, I won't stop you. If you say that you really want to do it,
The syntax of the sentences is obviously different.
The meaning is more or less the same from a certain viewpoint. If there is a difference, b) is more indirect, milder or more sophisticated rhetorically. Direct expressions are preferred to speak by barbaric and savage people, right?
Adults naturally tend to use such indirect expressions than kids for less-stressful communication, both in Japanese and in English.
I don't know about Chinese at all, but I think the same kind of thing happens to all civilized languages, right?