Do Peter Holden ' s Heart of England 300, I've ridden it about 10 times, and as Flaatuus notes - the first half of the first stage is the hardest stretch. There is a few km after Daventry that might get you cursing as well.
The Rough Diamond has a good climb on stage one, but all riders seem to cope with it fairly well, there's short rolling sections after the first control and again just before the second control, and the steady pull from Llangwm to Mynedd Bach.
The Rough Diamond is much the quicker route.