I agree with Dave... best four doors out there now.
My son has a 2010 MKS, I have a 2013 SHO... the only differences we notice besides minor sheet metal treatments are the huge MKS sunroof (nice!), the adjustable suspension (sweet, but expensive shocks!), the seats may be better leather with more interior wrap (for the extra 10K MSRP it should be), and the THX certified sound system (definitely superior to the SHO... has one big sub in the rear deck, but still needs subwoofer improvement). Using the decimeter, admittedly in my phone, no apparent noise level differences. All the other options mentioned here... seats, etc, are on both cars.
Go either way ... you will be happy! I have no regrets on the purchase... nor does my son