As a former Travel Agent, I'd say that you go directly with the cruiseline, it gives you a lot more control over your booking. Most travel agents don't have any extra pull to get you OBC above and beyond what the cruiseline is giving you already. The exception to this is if the agency has a group on that ship or if they are having a special promotion with that particular line. In reality, you are talking about a $50 credit for a week long cruise, pretty rare to get much more unless you book a suite or a longer cruise.
If you are really bent on getting an OBC, look at the national chains, they do the most groups and no you don't have to even know the other people in the group.
Andrew