Nikon D5100 DxOMark ReviewThursday April 28 2011 Sensor Review
Since the two bodies use the same sensor, their metrics should normally be very close, if not identical. The Sensor Use Case scores confirm this: they are almost identical.
These metrics show that D7000 and D5100 have the same sensor performance. The differences will be found elsewhere on the spec sheet: AF, body construction, memory cards slots, etc.