I think it's fairly safe to assume that the house broker for a stock will always have more influence than one of the others, Also the other major influence is of course an analysts Extel ( rating ) thus the nbr1 rated analyst who also happened to work for the house broker will have the most influence. But don;t forget unless you have a mate who works for the brokers, how on eathe could you find out about the upgrade/downgrade early enough to be able to trade on it?
You are technically correct that an analyst at a house broker would probably have more information than the others, but I would tend to be wary of their comments. I may be cynical, but I suspect that a house broker is more likely to put a positive spin on things. After all, they want to keep their client happy, and are therefore less likely to downgrade a client's shares, or place a sell recommendation on them. The recent Wall Street scandals relating to investment banking / broking conflicts of interest are proof that this happens.
You can receive near real-time releases of broker upgrades / downgrades from Thomson Financial First Call (www.firstcall.com). This is what the professional institutions use...and is therefore expensive. You can get the same info from 3rd part re-distributors, but they will be time delayed.
Private traders can access info from Multex (Multex.com)