If you are taking clients around town, you really should step up to the larger M5 and wear a Patek Calatrava.
The Rolex Submariner is too gauche (especially with a Suit) and the M3 is for the younger set.
I don't really have M5 money - I have a 6-year-old 330i and a 10-year-old M3 which I mostly bought as a track/weekend toy. They each have only about 40,000 miles so I am not looking to replace either in the immediate future. I really don't ever take clients in my car anyway. My partners drive a 2008 A8, a 2007 S6, and a ~2000 Honda CR-V, so my 330i puts me somewhere in the middle.
I most definitely don't have Calatrava money, nor do I love the design. I think if I were looking to buy a dress watch after a good year at work I'd buy a JLC Master Ultra Thin. Personally I have no problem wearing a diver with a suit, though I seldom don a suit. I have taken to using my Railmaster for all around daily wear, because it works with jeans or with a suit.