I stayed at the Maida Vale Marriott (formerly West Hampstead) last March. It's a nice hotel, large rooms, amenities, health club next door, decent restaurant and ice makers on the floors. Best of all, it's CHEAP. I've seen rates under 100 pounds sterling there. The tube is a block or so up the street. It's on the Bakerloo line so it's convenient to Paddington, Heathrow Express and central London.
It's not a nice a neighborhood as the Regent's Park Marriott one mile east but I'd recommend it if you want a convenient Marriott property on a budget.