Would the National Guard or Reserves provide more opportunities for promotions and advancements?

Keep in mind if you enlist, oftentimes you can get technical schools that are not available to officers - schools that will provide certifications that are very expensive to obtain otherwise such as cyber security. Enlisting can also lead to becoming a warrant officer, whom are commissioned upon reaching the grade W-2 and you can progress to W-5. Knowing you college major and minor along with career interests would be helpful.

IMO, it is easier and faster to get promoted in the Army National Guard until you hit the rank of Major. At that point, since in the National Guard you are only dealing with a single state's manpower, the ranks above that tend to have people "camped" on them, while the Army Reserve is dealing with positions nationally as long as you are willing to travel. It would also depend on your branch. The Army National Guard is Combat heavy, has a lot of Engineers, MP's and Medical units, all of which would generally promote on schedule through Captain.
The other "Advantage" to starting in the Army National Guard is that there are several option of how you go through Officer Candidate School, depending on the state. Most will let you attend the Federal Program at lovely Fort Benning, Ga., some have an accelerated State version of the Active Duty school, or you can attend an extended version that takes a bit over a year, but includes two Annual Training periods and then your monthly drills are at OCS for the remaining time. All require that you complete Basic Combat Training first. If you go Army Reserve, the Federal Active Duty course is where you will land. I have no experience with the Accelerated Version, but as a warning, OCS is not fun.

The National Guard is well known for being very difficult to advance in. Because it's a state force and quite small, you have to wait for someone else to vacate the position you're moving up into, which can take quite a while.

The reserves also tends to offer more support jobs, but this widely varies based on your area. Go talk to a recruiter, see what's even near you. If there was a guard unit 10 minutes away, but you'd have to drive 250 miles per reserve unit... I'm sure that would also impact your choice.