The best brand as a professional photographer is the brand you project to the customer - i.e. YOU.
Neither they, nor you should be paying any attention to which camera you use. I mean, no one asks you which brand of shoes you're wearing right when you go for a shoot? They just expect photographs well above average and professionalism in the person they will be dealing with.
Having said all that:
1. Professional level equipment from Sony and Nikon is not going to vary wildly in terms of output required for a job.
2. I'd suggest you choose a more popular (widely available) brand not because of photographic reasons, but more to do with service support, cost and availability of future accessories and possibly an easier to sell item in the longer term.