To the thread title, the benefits of a Master's Degree as an URL officer is a check-in-the box that can increase your competitiveness for O-4 and above given everything else is equal. Not exactly required, but many have one.
However, there seem to be a lot of posts that say "go into the Navy now, get your Master's later." Well, there's a little more details to that advice than that:
1-If you go into the Navy, it is likely that you will have the opportunity to earn your Master's on your first shore tour. However, where you get that Master's is going to be determined by the needs of the Navy. You are not guaranteed an NROTC instructor job at the college of your choice so you can get a degree. And you may even be assigned to a job that actually requires long hours such as a staff job working 100 hours writing emails and power points for a workaholic O-6 trying to make flag... not much time for a degree there, especially if you are married or have children.
2-Most of the Master's programs inside the Navy are good for fullfilling their purpose for your career in the Navy -- a check in the box for promotion -- but may not be so good for fulfilling any alternate career paths you want to pursue or at least keep optional. Besides, a large portion of the worth of your graduate degree is networking with prospective employers, something you may not be able to do because of point 1 above.
3-The GI bill can take a good chunk out of grad school costs, but it most likely won't pay the entire bill.
So given all this, if you can swing it, are looking at a good school, and are doing this for your employability as a whole, going for your Master's now will probably be the best choice. It simply leaves the most options open to you for employment both in and out of the Navy. It's also the only way you can guarantee attending a school tha you want in an area where you want to attend it. After you graduate, you'll be 25 and you'll have 2 years to apply for a commission. That gives you plenty of time to apply, especially if you start your first application in your last few months of school.
Of course, if none of the above is a priority to you, then go for OCS and attempt to get a Master's in your first shore duty.