Won't help the OP since the rugby game date has passed, but I'd bring both lenses and my 70-200/2.8. Use the 200-400 as the main lens on your best camera body, keep the 70-200 on your shoulder on a second body in case the action gets too close, and keep the 200/2 in a bag just in case the light goes bad.
You can't use a lens you didn't bring, and it's tough enough to get great images without knowing that you could have gotten this particular one with the lens you left at home.
