'Us' sounds a bit more natural, but either technically work. Ourselves is a reflexive of we/us, and it is technically the subject in your statement that the verb is describing, but to me it really doesn't fit that context. Alternatively, the statement may be better suited as "We expect it would be our decision." If I am understanding the original statement correctly.

By us is better, if you want to use ourselves you say: - We excepted to decide it ourselves.

I think "by ourselves".