A producer and a mastering engineer is more then just a choice you make because of locality or availability. Most good producers develop a relationship with their mastering engineer that is crucial to their sound. The two of them learn from each other on how to achieve an optimal sound. Unfortunately many times record labels, managers, etc. will try to get in the way of this relationship for a variety of reasons. This is where I have seen many records go wrong and run into problems. Choosing a mastering engineer based on quick turnaround or “having the hookup” can be weak link in your recording chain. Just as important as a mastering engineers talent in their ability to get your producers vision. This often takes times and some relationship building. Your record producer is always thinking about how to get your sound to be the best it can be and most often is obsessed with it. The business guys are often thinking about budgets and schedules. If you are really trying to make an exceptional record, it seems obvious who you should trust.