I'm not a runner, only cause I can't yet... heh, but I think they are both good for weight loss as far as I've read. The key is to keep your heart rate elevated in a good zone either way.
You will burn more calories in a short time period with running though so if you have a good fitness base you might look at running or a couch to 5k program. If walking very briskly is enough to get your heart rate up start from there you'll be getting your cardio geared up for running.
Half Iron Triathlete as of 7/2010
Training for Ironman Wisconsin!