Right now we're only cruising Carnival. We do two types of cruising... getaway cruising and vacation cruising. Getaway is simple: we live less than 30min from the port of Long Beach, so we'll jump on whatever 3- or 4-night sailing is convenient and inexpensive. (Just got home from a 4-night on Imagination two days ago.)
Vacation cruising is a different story. We generally have an idea of what we want to do (Alaska, Hawaii, exotic Caribbean) and wait for awesome cruises, especially 14+ nights. I'd been stalking Alaska for years when they announced a 14-night Alaska itinerary round trip from Long Beach; we jumped on that SO fast! Similarly, we've been wanting a Med/TA B2B and have almost booked them more than once but the timing was never right... when they announced the Legend for 2020 it was a *perfect* fit so we jumped on it. Tickled to death that we were able to get our favorite bowling-alley balcony cabin, which we also had on the Pride (15-night exotic Caribbean in 2015) and the Miracle (14-night Alaska in 2017).
We'll be looking for Northern Europe in 2021, so I'm expecting to have to look beyond Carnival.