Do you know which you prefer from a flavor and aroma point of view? I would choose your least favorite (especially if it is high AA%) for bittering (60 minute) additions, then split the rest up to maximize your flavor and aroma. It's going to be good regardless of exactly what you choose, so RDWHAHB!
Edit. I second what KGB said. For an imperial, I would use at LEAST 10oz up to a pound or more.