Don't follow the advice on what he said about colour - most bizarre comment posted in the thread. Focusing purely on colour, navy and charcoal are both perfectly acceptable conservative colours. (If one was going to be really nit-picky, I would suggest charcoal had a slight edge in pure CBD terms. Certainly this was a traditional view in the City, and I think Manton takes the same view in his CBD thread. But I don't think there is anything wrong with navy per se, I'm just mentioning this as I've never heard navy described as a preferred interview colour.)
Ideally neither would have pinstripes, but clearly that's not an option.
The only other thing that concerns me about the navy suit is you say it is slim fitting. If it looks fashion forward, then the charcoal may be a safer bet.