In my area, you have to be a Realtor to access MLS, but this isn't the case everywhere.
Aside from that, to me the answer is that although both are licensed to do transactions, working with a Realtor offers a layer of protection for the investor beyond that of an agent.
First, you can infer that someone who goes the extra mile and expense to become a Realtor is serious about his or her business.
Second, in principle there’s a level of trust associated with the name. Realtors are vetted for certain types of convictions and bankruptcies before they can join. We have to commit to a higher code of ethics than an agent, to treat all parties fairly and honestly, and to place our clients interests higher than our own. This may seem like a no brainer, but nonetheless, it’s part of the requirement, and if a Realtor acts against the client’s best interests, the client can take action against them, beginning with the local or state Realtor association.
If you’re asking from the perspective of becoming one, then the biggest advantage of being one is that it can give you some credibility right out of the starting gate.