Pros for HH: you can be credit-card-Diamond out of the box. If you play promos right, you can earn an insane amount of points-per-dollar on spend (I'm typically in the 50-60 range). Stronger in the mid-range and lower end hotels if you need those. (e.g., Hampton > Fairfield, HGI > CY, etc.)

Cons for HH: Everybody's Diamond, so don't expect suites. The program devalues often, so it's not a place to hoard. Right now I assume 0.6-0.7 cpp but know we could be headed to a nearly-fixed 0.5 within a year or two. Weaker at the top end. A few gems, but nothing like SPG/Marriott combined. A *lot* of mediocrity at the standard business hotel level. (I put Marriott/Westin/LM/Ren all > a mainline Hilton or Doubletree.)

Pros for MR: the program is just changing now, so if you are okay with what you see, it'll probably be stable for a few years. Marriott and Starwood both operated relatively stable programs for many years. If you can reach Plat Premier, you may benefit from some decent on-property treatment at heritage Starwood brands. I'm just hoping that some of their on-property culture is retained. Otherwise, it's similar to HH Diamond...but there isn't an out-of-the-box credit card path to Plat Premier. Lots of aspirational properties at the high end, although they'll cost you a lot of points.

Cons for MR: heritage Marriott brands have little/no upgrade culture. Can be weak in the midrange/lower-end, although as those brands are getting makeovers things are improving. (SHS, for example.) The Travel Packages are mostly dead in the new world. They exist, but the mileage-component is no longer a big value lever over the hotel component, so you might as well just redeem 5-night stays. The credit card free nights are limited to mid-categories, whereas Hilton's are close to unlimited.

Starting anew, I might get the HH credit card (they'll kind of pay you to carry it)...but prioritize Marriott stays if I felt like I could at least be a 50-nighter per year.