Unfortunately, there doesn't appear to be a single document specifying the similarities and differences between the two. However, as in the other answer; the package versions for SLES 11 indicate the current iteration of SLED is built from a heavily tested, cleaned, and the reduced 11.3.
One difference between the two is, as you questioned, the update policies. While OpenSUSE releases another iteration every 8 months (12.1, 12.2, 12.3), SLED remains on the same version for 2 to 3 years (10, 11), but receives a 'service pack' (a bundle of minor updates) usually around every 12 months (11 SP1, 11 SP2). There doesn't appear to be anything signifying SLES & SLED share a different code-base to what was 11.3, so any input on this matter would be appreciated.