The more I consider the pros and cons of each system the more perplexed I become...
Are you actually trying them, or are you looking at them on paper? Big difference, and we all have different requirements and different abilities, so sitting down and trying things (ideally, side by side) is the only way to find out what best suits you personally.
that person is clearly very bright, interested and knowledgeable
I look at it from a completely different angle.
The more talent one has got, the more he can achieve, and the less important the keyboard becomes.
Whatever you do in life, the outcome is a function of your natural talent and the time that you can invest into the activity. Results of 1hr of practice for an extremely talented person = 10 hours for somebody with very limited talent. And if world-class accordionists practice, say, 6-8 hours a day, a schmuck like me will need to free up 60-80 hours every day to get to the same level. Gifted people would always tell you that hard work & practice is the key, but they often don't realise that their 1hr is not equal to somebody else's' 1 hr.
None of us are going to live forever, so all of us are subject to time constraints.
The lower the talent, the more time you need to invest to get the same result (and the requirement grows exponentially). Any adult reading this message can, in theory, become a chess grandmaster...But almost nobody would have enough free time in their remaining lifespan to achieve it for their talent level, unless they are gifted with exceptional, Bobby Fischer-level talent. That's why an adult starting chess late in life & getting a GM title is almost unheard of. Nothing to do with not being bright, interested or knowledgeable, but everything to do with time.
Ticking away, the moments that make up a dull day. It's all about time.
Worked example: you've got one keyboard where you have 1 fingering pattern for any root note, and another keyboard where you need to learn 12 patterns to cover all root notes. If you have exceptional talent, the extra 12 patterns might be easy to memorise and might require only an extra 50 hours of practice. If your talent is close to zero, these 50 hours become 1000 hours of extra work. If you have a busy full time job, a family, you're trying to keep fit and have a couple other hobbies, these extra 1000 hours easily translate into extra 5 years that you'll need to practice an "inefficient" keyboard to reach the same level, as you'd achieve on the "efficient" keyboard in a year.