jrthomas is correct that the ELSA must be build exactly per the plans. It can, however, be modified after original certification without the manufacturer's approval.
One thing to note is that Kitfox has chosen not to market an ELSA kit. To certify yours as ELSA, you would need for them to provide you with a statement of compliance for your kit. (21.193(e)(4)). I don't know if that's something that they do.
ELSA advantage: Future owners can take a repairman course to do the annual condition inspection.
E-AB advantages: Possibly easier certification. Can be modified outside of LSA restrictions. (although, as WWHunter points out, once this is done, it can never be an LSA again.)
Basically, the differences are so minor, that the path of least resistance is probably your best bet. (E-AB)