Which camera is better for professional photography, a nikon d700 or canon 5d mark ii?

I started a photography business about a year ago. Now, I have been asked to do a few weddings and big events. My only problem is that I don't have a camera that works well in a low light setting. I'm looking to buy a camera that will deliver when I do weddings but also will be perfect with children, seniors, family and couple's portraits. My budget is $2000 can even go as much as $2500 but, no more. I have been looking at the nikons d700 and the canon eos 5d mark ii. I currently own a nikon d5000 and it has been a really good camera to me but, I need something better.