I have used
Dreams Unlimited and had good service. I have not used Small World, so I cannot say anything about them one way or another. I just know both offer similar on board credits for booking.
A lot of people use Costco as their travel agent. Instead of an on board credit, they give you a % of your cruise fare back in the form of a Costco gift card. The % back is a lot higher than the on board credits offered by the other travel agencies.