The Specialized Amira will be more stiff, if you're feeling like the back end is wobbling, but realize that it also has more aggressive race-geometry and may be less comfortable on those longer rides.
If you're not completely heartset on WSD, maybe try a small Specialized Tarmac or Roubaix, which will be more beefy on the carbon layup.
-- gnat!
Windsor: 2010 S-Works Ruby
Pantysgawn: 2011 S-Works Stumpjumper 29er
Whiz!: 2013 S-Works Crux (Singlespeed)
Boucheron: 2009 S-Works Tricross
Haloumi: 2013 Tern P7i
Kraft: 2009 Singlecross
Gouda: 2005 Electra Betty
Roquefort: 1974 Stella SX-73