such a massively situational item. Sometimes it is just obvious, and am assuming that the question is about those times when either way feel to be a viable option.
In that case, I'd look out a few years and the needs for when the venture is at least 10 times the present size. Do we want this partner, or any partner, or cash, or ... fill in the blank, at that future point? Answer that and the choice can become a little bit easier to see