Generally no, a cruiseline will not be the better deal. However, there are some people who have received better deals through the cruiseline than a TA, but I suspect they haven't found the right TA yet.
A TA is given a discount from the cruiseline for the fare which a portion of that discount is usually passed on to the customer. This is why you might see different prices from different TA's for the same cruise and cabin.
This morning I received an email from my TA who I requested a quote from and he was $100 pp more than what one of the online agencies is quoting. Don't know why, but this is the second time he has not been able to match or beat the others. I'm beginning to give up on him. His price is close, but less than (about $15) than the cruiseline's online price. Still, the online travel agency is $100 pp less than this.