You original question has to be on of the very best for a long time, just look at the quality of responses.
Over the past week I have continually thought if I were in your position, what would I really do?
Despite that I totally hate all Volvo engines, they still work and have a global back up. I don't like the locker seat height in the cockpit of the 54, so I would buy a cushion. The centre line aft bunk, well I can sleep forward when it's bumpy or noisy ( I always slept on the passageway floor to the aft cabin on my SM when on passage) . The defective main mast on the early models, I would ensure the factory repair had been carried out. The bloody table stopping the engine room door from lifting, there are several ways around that- or I'd just live with it. After all, the less I would have to look at that Volvo the better.
So after 30,000 miles sailing the SM and around 12,000 single handed would I buy the 54?
1. The double anchors. 2. The broader beam aft, giving more positive buoyancy meaning you really can carry proper davits and solar panels with worrying about dragging your stern. 3. No more temptation of setting those
downwind double poles when sailing alone. 4. Having sailed in company for 5/6000 miles with two other SMs and three 54s, the 54 is definitely a little faster but not much. 5. Despite my comments about less stowage, there is far more than you will need. If you look back you will find many posts about live abroad cruisers having to repaint the boot line on a SM because they carried too much junk .6.The bow thruster on the 54 appears to be a lot less hassle, I don't know this for sure. 7. The forward facing nav station and the generous layout for mounting instruments.
8. The washing machine and dryer in the fore peak, mess and smell out of the way.9. It's a more modern design and is so much brighter for living aboard full time. 10. Maybe the most important part of your question, ROC when it comes time to cash out, I would have thought that your 54 being younger etc. might stand up better and gibe more Return On Capital.
All I can say is, I wish I were in your shoes and had that problem.
Chantier Amel respect del la mer, there is nothing better.
Good luck & fair winds.