I'd say that Geneseo's course is slightly faster but not by an appreciable amount.
Yes, UR's course is in a generally flat park, but a lot of the little uphills that you run over multiple times can really mess up your rhythm and slow the course down overall. And also the grass isn't the nicest in spots, and there is some awkward sloping of the ground. I never found the small sections of pavement to be much of a help. I do agree that it's generally a fast course, but by no means one of the fastest.
Geneseo's always seemed faster. Flatter than UR and easier to get in a groove. The only downside I noticed was that the grass could be a little long at times. That slight uphill along the road was really nothing.
For what it's worth, I've raced at Geneseo 5 times and at UR 7 times.