They're both important, and if possible I would, and do, have both. But if I had to give one up, it would be the pressure gauge.
You already have an idiot light which is not perfect but something. If you lose that much OP, your engine is probably due for a tear-dowm anyway, or it won't survive.
What you don't have is a way to tell you when it's OK to run hard... say above 3000 RPM's.
You are in cold country and to me that makes it even more important. You might have water temp long before you are up to operating oil temp.