Depends partly on where you want to be. The Hilton is up on the hill on one side of the river; the Marriott is down by the major shopping areas. We loved both, but I'd have to give the edge to the Hilton because of the magnificent view we had of the Parliament building across the street. (That's an upgrade but it was well worth it. They had a fireworks display the night before we left!)

Great food in the Hilton lounge (never used the Marriott one). Hilton concierge found me a skillful, English-speaking dentist on a Saturday AM when I had an abscess, and the place was staffed by young women in white uniforms with thong underwear so my husband didn't mind hanging out in the waiting room at all!