I am moving to Poland in September to obtain a master's degree through a Fulbright fellowship. I have heard conflicting things about visas. Some people are saying to apply for a national visa beforehand, some people are saying just to go there under the 90 day entry for US citizens rule and apply for a residency permit once I am in the country. Is there any advantage to doing one over the other?
You won't get conflicting information if you just contact the Polish embassy or consulate nearest you and ask them what to do. That's their job. Make sure that you have all your scholarship papers with your, including the invitation from the host university. You'll need your US passport, and your university transcript and diploma if you have one. You'll also need proof of health insurance for the time you will be there, and documentation from the Fullbright Foundation about the financial particulars of your grant. You'll also need some more paperwork that they will tell you about, and inform you how to get it (you can't get it actually; those documents are not used in the US. But the embassy will tell you how to get the equivalents). Of course, you'll need four passport pictures, too. Take eight. Do this as soon as possible. Polish bureaucracy can be VERY slow.
DO NOT just enter the country and apply for a residence permit once you arrive. I did that and it was hell. Things are worse now because I just had to leave Poland one day and come back the next with exit and entrance stamps issued on two different dates. I crossed the border at Cieszyn at 11:45 PM, and came back after midnight. With Schengen, that is no longer possible. And more dangerous, because if you have problems, you can be deported and banned from ALL Schengen countries. It's a lot safer to take care of everything while still in the states. It will also save you a lot of time running from office to office getting all the papers you need for an extension on time when you are in Poland, which can be overwhelming even if you speak Polish.
So look up the nearest consulate or embassy tomorrow, and make an appointment.
DO NOT rely on websites for information, even the embassies own website. Poles do not use government websites for information, and the information on them is often outdated or simply doesn't apply in your case. Talk to real people on the embassy or consular staff only.