Another off site forum, dedicated to Sony/SonyEricsson phones and Android Smartphones by Sony is Esato ... most o the users there are from Euro nations, however they still provide some good insight over long term use of the Xperia S, and note that the Ion will be ready later this month on AT&T.
EDGE/HSPA/LTE (700/1700/2100 LTE),
Larger Battery than "S"
4.6" LCD at 1020x720 (HD)
12MP Camera (1080P video)
1.3MP Front Camera (again HD video recording @ 720P here)
MicroSD slot for storage expansion (the "S" does not have this)
Removable Battery (the "S" does not have this, supported but removing internal screws makes this possible)
DOCKS are compatible with "S"
I say check out feedback on BOTH units as I suspect the full retail or contractual pricing maybe VERY close (set $80 difference).
Formerly: Prom1, now I'm Supa_Fly!! My Feedback Mah Na Mah Na.