Advantage Marriott:
- Better, more interesting hotels (obviously depending on what you're looking for)
- Far better programme
- More reliable earn/ burn
Marriott issues to be aware of:
- There are significant issues with the SPG merger and the IT systems are currently in meltdown
- With 30-50 nights you won't get at the top of the Marriott elite programme, limiting your freebies and upgrades
Advantage Hilton
- You can get top tier status with a credit card, assuming you're a US resident (this devalues the top tier, but for you it's better than not getting it)
- Programme is currently stable and there are some good value redemption opportunities out there
If I were you I'd look closely at the hotels. For me, Hilton simply doesn't have enough interesting hotels and so doesn't win over Marriott. If Hilton hotels work for you, then if you can get Diamond status you might find yourself better off with Hilton.