You get the same perks whether thru
DCL or a TA. That is, you can pick your stateroom--if you don't specify, DCL will suggest one. You can accept or reject that. You can ask for a specific area of the ship or for a specific room.
I like booking thru DCL, then transferring the ressie to a discount TA. That way, I have the best of both worlds.
Another reason to use a TA is that on one occasion, I was "double billed" for transfers. That is, they were a part of my package, but then DCL billed me again. It was nice to have the TA to dispute the matter with DCL. In fact, when DCL wasn't as fast in refunding as she thought they should be, the agency sent me a refund and then continued to fight with DCL so that they could be reimbursed. It's nice to have someone on "your" side in such cases.
The downside to a TA is that you can't check your ressie on the DCL website. However, the discount I get is worth that minor inconvenience.