Personally, I feel like 'therefore' is used for everyday use. I only say 'hence' if I am arguing with somebody and I want to make a point (maybe not always argue, but a more serious topic). I use it if there is already context to make it short and sweet. For example, if we are talking about Jimmy falling off his bike and scraping his knee, if the context isn't already there, I might say, "Jimmy fell off his bike, therefore scraping his knee." But if both parties already know Jimmy scraped his knee but one doesn't know how, I'll say, "Jimmy fell off his bike, hence the knee." Obviously, if you use it interchangeably it's fine. But from my experience, this is how I say it.

I think you can use them basically the same way. Except "hence" can sometimes sound old-fashioned. "therefore" is more common.