You will always get a better deal with a TA, although you should shop around among several agencies. We book about 5 or 6 longer cruises a year, and use a personal short list of 5 TAs that we can check on-line and via phone. With RCI, there is very strict price controls (from the cruise line) which make it interesting. Many TAs will add amenities since they may not be permitted to drop the price. Sometimes you can find things like pre-paid gratuities, on-board credits, etc. Actually, the most interesting deals are with Azamara (owned by RCI
) where we recently took an 18 day cruise and were able to get $900 in on-board credits (a record for us). It is against the board policy for me to list TAs, but you should be looking at the larger national TAs.
By the way, the reason we say to use a TA is that the cruise lines will not match the amenities (and sometimes group rates) that are offered by TAs. One warning about TAs is that most of them now will charge a concellation penalty (can't blame them for not wanting to work for free) so you should always ask about this before booking.