I'm not sure if things have changed but I was under the impression that Royal Caribbean didn't do heavy discounting via TA's. However, TA's might be inclined to throw in different amounts of on board credits. We checked out Cruise Compete a few years ago where TA's kind of "bid" for your booking with different perks.
Apart from that, I'd recommend signing up for their marketing emails.
From a status matching perspective, I think they match status from Celebrity's loyalty program.
The only thing that pops to mind between a 3 and 7 day cruise is the number of formal nights which should be 1 and 2 respectively. I tend to say that the formal nights typically have a more upscale menu.