I am an investor and I can tell you about my preference.
If I were only buy and sell 1 property at a time and probably only this property within a few months of time, then I would use one realtor. This realtor would be someone who I have established a good communication, and we both on the same page about what we want to accomplish together.
If I were to buy, sell, hold several pieces of properties at a time for deals involving short sale, bank own, private sell, rehab, rental etc. then I would not be hesitate to establish relationships with multiple realtors as each one of them specializes in a specific type of transaction.
If the deals are only a few, work with one realtor for loyalty and effective communication purpose.
If there are many deals, share the opportunity and share the wealth.
This is how I manage my business. :-)