The only real way to know for sure which program and which brand you prefer is to spread your stays out with both. I maximized points with both programs by getting both credit cards. I don't stay in hotels enough anymore to get the top tiers, so I compared my stay habits, the types of properties I like to stay in, and the things I find important (free breakfasts over late check outs).

I am a Hampton/ Hilton Garden type traveler, so I don't really value Diamond room upgrades that much. I do value a free breakfast at Hilton Garden or Hiltons. When I stay at a Courtyard as a Marriott Gold and still have to pay $2.50 for a cup of coffee in the "Bistro", I find much less value in Marriott vs Hilton.

But your experiences, stay patterns, and preferences will be different.

You have the unique ability right now to attain high tier status with both programs, so its a perfect time for you to try both and see which one works for your particular preference. Consider getting branded credit cards for both, you will really start accumulating points and free stays.