Why might one say that the ESV didn't use as much new scholarship as the NRSV? I've read lots of reviews, but not found any good summaries. However the following brief comment expresses something I've seen fairly widely:
Rawson Street: NRSV v. ESV. The NRSV has done a better job of fixing up misleading wording. But there is also a theological difference. The ESV sticks with Protestant tradition even when many feel there's good reason to change. This is where I'm finding a hard time getting a good summary. It's the kind of thing that I (and possibly Unix) typically encounter when looking at individual passages. The ESV isn't nearly as bad in this respect as the NIV. For that I have documentation:
The New International Version (NIV) - A History and Evaluation. There are also systematic differences in the OT, with ESV tending to show passages as understood by Christians, and NRSV showing them as intended for the original audience. This is not a denial that the OT refers to Christ, but rather understands that they also had references to things at the time the prophets wrote. E.g. Is 7:14, in addition to being used by the NT for Christ, clearly referred to a woman at the time. It is unlikely that that woman had a virgin birth, nor does the Hebrew suggest it That doesn't deny that the passage is also legitimately used to refer to Christ.
My preference in both respects is the approach of the NRSV.