@DudleyDog73 - Had to google how Princess handles their discounting these days which brought up CruiseCritic thread
http://boards.cruisecritic.com/showthread.php?t=1801857
I'd expect CruisePlum to show current widely available prices matching what the cruiseline's direct booking website currently shows and consider that the deal to beat, if I were hunting solely by price and perks.
I've been sailing with RCCL corp ships more recently which doesn't allow quote discounting anymore. The largest recent discounts I've noticed via my usual TA for Royal Caribbean and Celebrity have come from TA making a large group booking when prices are low and reselling the cabins to customers over a period of time after prices increase. The TA has a high volume of Celebrity and RCCL customers so they can usually make their numbers work out. The cruise line also occasionally offers senior, resident, and military fares in specific categories for limited time which I spot check via cruisefish.net (also tracks long term historical pricing per cabin subcategory). I wouldn't expect CruisePlum to reflect the TA promo price, as that is unique to the TA.
@Nanook - I've yet to see airline point cruise portals beat what a well motivated cruise travel agent can offer in on board credit or cruise fare discount but I'm sure this differs per line and sailing (Disney cruises seem to have consistent pricing and few commission rebates). I personally prefer to save money especially since cruise travel agencies and cruise lines qualify as 2X travel in CSP resulting in cash savings and points
I'd also check what the "cash out" charge is from the casino for remaining non-refundable cruise line related OBC vs the value of the points and take the better offer. In the Disney example, they don't have casinos but I've read it is possible to purchase a Disney Gift Card with on board credit. That can be used to purchase Disney tickets or season passes as well as merchandise or resell the gift card. If the per room stateroom and OBC is consistent then I would probably choose between cashback for the online travel agencies like expedia/orbitz/travelocity, points portal, and handling the booking directly to retain control of the reservation.