Being partial to Marina Militare dials, I say get the 339. The Destro in Titanium is amazing, and I had the opportunity to purchase one but went with a 217 instead. I couldn't be happier. I also have the 339----> MM dial, composite case, SE of 1500, 8 day, Giorni Brevettato.... A true future grail, IMO.
What's not to love?
My gut says 339 will be the watch, but the 390 'fits' into your line up nicely with your 372 and 232. Gut wins, go 339!
"Life's an adventure, you might as well wear a Panerai"
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All are awesome choices and seem to compliment your 232 and 372 nicely. The 390 has me in its grip right now so that's my vote...even though that means there might be one less for me to get my hands on.
I own both. I love them both. But for different reasons. The 339 is so unlike any other Panerai. You will be surprised how it wears on your wrist - so light, so unnoticeable. The assolutamente is thinner than your other straps. It's just a different experience.
The 368 is just drop dead gorgeous, but a traditional Luminor look. You have that covered with the 372. There will be other Ti destros. I don't think there will ever be another 339.
To me, the 390, is easily duplicated and you already have the two best and most DNA in 232 and 372. So it's your call as to either the 339 or 368. If it weren't ceramic, they only thing that would enter my mind as a knock on the 339, the 339 would be the no brainer. But the 368 is a cool piece and it's an 8 day great movement. Tough call, but follow you're heart. Me, I'd probably opt for the 339 after arguing which myself about the ceramic.