USAA serves their members well. I have not used their new car negotiating service, but have heard great things about it.
With regard to buying a new car locally so that you can have the vehicle serviced where you bought it, I really don't think it makes much difference these days. In most cases the service department may not even be aware of which dealer you purchased the vehicle from. The only advantage would be for immediate post purchase issues which are not covered under normal warranty... such as paint dings, etc. In those cases, the selling dealer would be much more likely to make you whole.
2010 Forester Premium Paprika Red Pearl