Thank you for the advices.
I find it very annoying that so many Marriott hotels in Europe are located in remote areas.
There are many hotels with a much more central location which are also cheaper, so I don't see any reasons to book with Marriott... apart from a few MR points that I don't really need.
I chose the Lisbon Sofitel, which is no.2 on TA's popularity index (Marriott is #23) and I also got a cheaper rate with beakfast included.