Stating belief is more rational . . .

[Watson]

Because of the way rationalisation works. You begin with a conclusion and then induce supporting argument for that conclusion;

This is perfectly true. You begin with your conclusion based on understanding and knowledge, which then allows you to work backward and see the supporting arguments for what they are.

also in other contexts such as psychology, rationalisation is considered a defence mechanism. Saying I believe in God's love, a promised heaven, and human focus because I don't like the alternative.

I don't know about this, it all depends on the conclusion being drawn. Drawing the conclusion "God exists and loves us" gives way t supporting arguments which actually discredit the alternative entirely. But I suppose in a sense, that is a defence mechanism.