Go to Gomez and Molina in Marbella by the fountain gardens and you will probably see a Sub in stock in the window. They had two a few weeks ago when I was there. Most Spanish guys either have a Seamaster or a Sub and sometimes a GMT2. Lots of TT Subs down there and Bling Bling Day/dates!
The main differences of an SD over a Sub are: Thicker case with HE valve, Thicker Bezel and Crystal, No cyclops, Deeper case back, Stronger bracelet with stronger end links, Different Extension link on the clasp. I think that covers the main visual differences.