Here's why. Even though I voted for the X5, (it's the look, not really the sense of security factor for me, notice how it's very petite women that go for huge trucks), you made a strong point when you said that you plan on having no children at least in the near future. A woman can always cherish her moments now when she is young and able to (i.e. her age looks good in a convertible. Her marital status w/ no kids is great for a convertible). But this can't be the other way around when she is pregnant, or has an infant seat. Picture: She's nine months, and she's stepping out of a convertible. It looks like she's riding the freedom wave till it fades.
Personally coupes are always a surefire thing, but hardtop convertibles are the best. And if the weather permits for a more pleasurable drive with the top down more times out of the year than say, New York, then go for the convertible.
There is nothing more sexier in the world than to present a woman with keys to a brand new ride as a gift. Screw Diamonds, it isn't for everybody and not all of us care for jewellry (trust!). She'll thank you a million times in one!