I don't have enough direct comparisons at Marriott. I'm only Gold, so I know I'm probably not getting upgraded either way, and use OLCI whenever it's offered. I don't wait in hopes of an upgrade. A nice thing about Marriott is that I'll sometimes get the "your room is ready" notice very early in the day.

With Hilton, I've been directly told that using OLCI freezes you out of the upgrade pool. I was trying to OLCI the day prior, and I had a basic question about a room. So I phoned the hotel, and the clerk said "Don't OLCI! We run our Diamond upgrade process the morning of arrival at 11AM. But we cannot touch a reservation after OLCI has taken place. So...stay out until noon tomorrow, and you'll have a suite." Sure enough, that worked. There's a long thread on the HH board where the consensus is that they *can* touch the reservation post-OLCI if they really need to, but generally don't during normal operations.