I haven't done a sea cruise, other than on a freighter from Shanghai to Hong Kong back in the early 90s: not luxury! I've also done overnight trains through Asia/India/etc which are a great experience, but nothing luxury about them (triple storey bunks, people sleeping in the aisles).
The train/cruise trips I linked have been free/contra as I'm a photographer, but in each case I've largely had the same experience as a passenger, just without feeling a need to measure my enjoyment against the cost. The Kimberley trip with Coral Expeditions is comfortably at the top - you're in and out of inlets and amongst islands so the views are always interesting and changing, the landings via Zodiacs are to beautiful/historic places. Catered meals, drinks with views, cloudless weather every day, adventurous and interesting people to meet and eat with at meals. The optional afternoon lectures might be about exploration history, indigenous art, animals and environments, by seriously experienced guides. There's more variety in strolling the ship - bridge and engine room tours as well. And 100pax is a great size because you know everyone by name by the end, unless you've tried to avoid it you've had a meal with everyone including the captain, etc.
Of the Journey Beyond trips, Great Southern is the best of the two I've done. Same train as The Ghan, but it's a newer trip so the off-train wine/dine is ramped up and impressive (we photographed the second-ever trip and had the designer of the route/locations along with us). The food and drink is relentless - you eat and drink at wineries, lunch in an old gaol, out beside the beach, amongst mountains, etc. On train, you're typically in the dining carriage (2-4 person tables), the bar/lounge where you can meet other passengers, or in your small room. The base cabin is a couch which turned into a top/bottom bunk; higher class option has a queen murphy bed. On those trains, not a single window in the passenger areas is openable, so it's a bit stuffier.
The cruise rooms were far larger, king-size beds, excellent en suites, a completely comfortable and usable desk, balcony, etc. Lying on the bed with a book or working at the desk with the balcony doors open to the breeze and passing ocean was just serene. At meals, there were tables for 4-10 people and you'd just ask to join any available spots; even for an introvert, you inevitably meet interesting people. The sorts of people who are on a $1k/night expedition cruise are typically well-travelled, have interesting jobs, etc. Over 60 meals, even the dullest people I sat with were a decent, friendly couple who I was happy to talk with.
The rail trips are great, but feel a bit more contained and herded. 2-3 nights feels like it's over quite quickly too. Both formats typically attract older clientele, though there were solo 40-50s on our trips, and I find all ages fun to engage with anyway. Meals on the rail trip were probably better; The Ghan actually does a nice thing where the meals are relevant to the area you're passing through. So as you leave South Australia you're eating seafood but by the time you're in the Northern Territory, it's barramundi, macadamias, mango, buffalo, etc.