I'd say "I think it's not true" would be used when you are sure that it's false. And "I don't think it's true" would be more if you just kind of think that it might not be true.

In other words I'd use "I think it's not true" if I'm more confident that it's not true.

大学受験してない奴はreadingが全然出来ないので、この程度の英文も読めない。

ﾗｸｯﾍﾟ MM61-3MvJ みたいな低学歴が背伸びしてもみじめになるだけだ。