To state the obvious, they're two VERY different boats which is probably why you didn't get much of a response the first time.
I've sailed Lagoon 440's and on a 450, as well as spending a lot of time around a couple of 380's. We sailed on and off with one across the
South Pacific a few years ago while on our Amel SM.
If I was going to do the
Caribbean again or the 'Coconut Milk Run' to NZ or
Australia, a well equipped Lagoon 380 would be high on my list due to the living space advantages and relatively benign sailing conditions. That said, a well maintained SM would probably still win out for me because I know them so well, I think it would be easier to
single hand, and the layout worked so well for us.
For continuing on to the Southern
Indian ocean, I'd definitely lean more towards the SM because of the reputation for much stronger tradewinds and relentless large beam swell friends have described......but that's my preference. Others will point out numerous smaller Lagoons and Leopards that have done it
safety and they're correct. It's a personal choice.
As for the concern about 'all the systems' on the Amel, I'm not convinced there'll be a significant difference once you get a Lagoon 380 fully equpped with
watermaker,
solar systems, high output alternators on the engines,
autopilot (including installed backups or extensive spares) etc. The only additional systems you've got on the Amel that I can think of are:
-Electric furlers (3
electric motors & gearboxes that I rebuilt at the start then probably spent a couple of days in total 'maintaining'...replaced a couple of
brushes and had one
motor rebuilt for $75 in
Fiji..... over the next 3 years). Way more than offset by not having to get off-watch crew up in the middle of the night for sail changes!
-
Air conditioning (but some Lagoons 380's also have this)...... we used ours for brief periods at docks in
Grenada, and I ran it once a month to keep the system operational. If it had crapped out, I wouldn't have repaired it as we preferred being at
anchor and almost never used it.
-
Washing machine..... Any
maintenance there is more than offset by the time finding laundromats in new
ports. With a
family of 4 we loved the convenience (luxury) of having it onboard. Not necessary by any stretch though.... so it's a maintenance item if you choose to keep it running.....
- Generator.
Engine maintenance would be the same as the 2nd
engine in the
catamaran. The generator is potentially more maintenance, but the Lagoon will still have a
charger and
inverter anyway.
Finally, keep in mind that most of the long-term cruising
boat systems (ie. high output alternators, backup autopilots, watermaker, even downwind sail setups, large
solar system,
SSB if you decide, etc.) are custom to the Lagoon and will vary in how well they're designed and integrated into the boat. The only one on that list that's an add-on to the Amel is the solar system. The rest are part of the package and were very well engineered both in how well they were integrated together, but also for maintainability. That does make a difference in maintainability and
reliability in my opinion and could easily close the 'maintenance gap' you're suggesting between the two.