Wow talk about a post landing right in your wheelhouse.. I have been a hotel GM for over 25 years. Vast majority of the time with Marriott but last 9 or so with Hilton. All different properties and locations.
First off, both really are great.. Your DD won't go wrong with either one. Many issue you come up with are related to the specific property and not really the Brand.
I found Marriott to be great for really "hand holding" you on everything. We used to kid around that there was a SOP (standard operating procedure) for you to pee. They are that detailed. So there is appeal for a younger person to work in a strict program, learn good fundamentals and a solid base. Hilton is a little less strict but that also allows your personal style to grow. Hilton is also a little more into you expressing yourself with the Guests and building rapport..
Again, you won't go wrong. Both will be good. What really comes down to is your hotel GM and how he/she is. That person will be a huge influence in your DD intro the industry. If there are specific questions, I will be happy to try to help.