In answering your question, I guess it really depends on how many watches you already have and where you intend on wearing your new watch. The Rolex Datejust is a very practical watch and simply indestructible. I have worn my Lady Datejust (stainless steel/18K yellow gold jubilee bracelet, 18K yellow gold fluted bezel, ivory Pyramid dial, Roman numerals) at work, playing tennis, playing golf, swimming, hiking, jogging, to museums, on dates with DH, to the pumpkin patch with my toddler, etc. It looks appropriate with a suit, daytime dresses, jeans, and exercise/outdoor attire. Every five years or so, I take the Datejust in for a complete overhaul or service. For dressier occasions, I have a dainty vintage 1940s diamond and platinum tonneau Hamilton with a simple diamond bracelet.
While it does not keep time as accurately as a watch with quartz movement, the Datejust's versatility (and the fact that I need not worry about replacing a battery every year) has made it a great everyday watch for the past 14 years. Both the midsize Rolex Datejust and the Cartier Roadster with the pink face you are considering sound lovely. However, if you had to start with one watch, I suggest the Rolex Datejust as it is just a great basic, the watch equivalent of the little black dress or a simple strand of pearls. If you get the plain bezel Datejust I would probably go with the Oyster bracelet (rather than the Jubilee bracelet) as watches with that combination seem more stylistically coherent with a cleaner, more elegant look. The Cartier Roadster with the pink face would be a great second (or third or fourth or fifth...) watch to get if you want to add some fun and variety to your watch wardrobe.