Have you had a leather strap watch before? I personally hate them... they get dirty surprisingly quickly if it's a daily watch and cost so much to replace. For that reason I'd do two tone for sure. But if you're used to it then maybe it's a moot point.
I would always choose the bracelet option over leather. I like the look of leather band watches on men sometimes, but they get dirty and look worn rather quickly, plus they just aren't comfortable for me.
I think leather makes more sense on a more casual or sporty watch. The BB is an elegant design and that's why I think the bracelet compliments it more. But you can certainly do what you like!
Also depends on how you plan to wear it. If it will be a special occasion watch, black leather would look very nice. Or if you love the look and are prepared to replace it regularly, go for it!
But imho, I have to agree the Ballon Bleu looks best with a bracelet and SS rather than 2-tone best.
I prefer two-toned bracelets vs leather. Leather gets sweaty and sticks to my arm in the summer. You can always get the bracelet and buy a leather band at a later date and switch them out!