I've stayed at the Metropole. It is a big hotel with big hotel amenities. It treats Hilton Gold / Diamond members well. There are some rooms at the Metropole with 2 US-type double beds. So, you could fit in one room at the Metropole if you book the right room.

IME, the Doubletrees in London are a crap shoot. Some are good. Some are bad and don't care (West End - tiny rooms; poor Hilton VIP recognition). I haven't stayed at the Doubletree Westminster.

Westinster is probably going to be a more touristy and expensive area and convenient to Victoria Station and travel to the south. Metropole at Edgeware Road is in a more ethnic / Middle Eastern neighborhood, although you can walk to some touristy stuff, and more convenient to travel to the north via Paddington, Kings Cross, and St. Pancras. It is quite possible to take the Heathrow Express (or Heathrow Connect) from LHR to Paddington and walk a couple of blocks to the Metropole. That is a lot easier than dealing with luggage on the Tube.

Bottom Line: the Metropole is one of the hotels I stay at in London and probably the one I stay at most often because of the combination of price and location and amenities. But, YMMV.