Janeseattle- Welcome to Autopia!
As
velobard said, the differneces between waxes (and between major companies like
Meguiar's and Zymol) really aren't all *that* significant- it's pretty much a coke/root beer sort of thing. Where Zymol has its *real* reputation is when it comes to their exotic "Estate Glaze" line of waxes and that's a whole 'nother league from what we're discussing here.
IMO the biggest challenge is to keep a black car from looking all scratched/swirled/marred up- if it has a zillion tiny scratches it won't matter *what* wax is on it. If you concentrate on washing without marring the paint (tougher than one might think) and prepping properly before the wax (that's where the polishes and paint cleaners come it), you'll have a great looking BMW.