They are both the same with different names simply to deliniate between the rating being held by a NPPL holder and a PPL pilot.
No they are not, one imposes limitations that don't exit in the other. For example: A max weight limit and with SEP you can also fly a Microlight; with SSEA you cannot! Article 28 requires that you have a valid C of E or C of T for any rating you are exercising; it was only amended this year.
If your privileges are limited by virtue of this Exemption then you need the appropriate rating in the licence to exercise those privileges unless they issue an Exemption to Art 28.
If a pilot turns up for a revalidation with a SEP rating and is operating an SSEA the examiner could in theory revalidate the SEP rating based on the JAA criteria (no medical certificate is required) but the pilot is now required to meet the NPPL criteria and the Examiner cannot revalidate the SEP on the basis of that i.e. two half hour dual flights nor enter an SSEA revalidation if the rating is not in the licence!