Whether to swiftcasts vs. hardcast shadowflare in favor of a potential raise is all situational decision, there's no concise answer that can't be refuted. It would look more like a flowchart with all the variables to take into account.

One thing to consider I'm not sure I saw mentioned- the longer cast time of shadowflare carries with it a higher chance of the need to dodge and break that cast... completing 90% or more of the cast and then having to cancel in a situation where a faster casting spell could have finished is not something you want to have happen. So it's not just about shadowflare going of off a bit faster, it's also about being able to get it up with success even if having to dodge at the same time.