As a Marriott Plat and soon to be HH Diamond, I'll offer this:
- I've had better luck with Hilton in Europe
- Marriott lounges tend to be better stocked than Hilton
- I prefer the Hilton lower end properties to Marriott lower (HGI > Courtyard, Hampton > Fairfield).
- Hilton makes it much easier to maintain status with stays and nights, I frustrate the accountant with 4 invoices for a 4 night hotel stay

Hard to say which is better overall, but the big thing that sways my choice to Hilton is that they are generally 20-40$ less per night than the Marriott around airports.