I figure that if a lanky 5"-5" tall, 110lb, 13-year old had no issues reaching over on a MIII system on a huge Morino VIN, then it is a matter of simple preference and not so much physical restriction, but a simple matter of practice and getting used to it.
That said, because I started with the MIII C-system, it is going to be my obvious preference. One thing that I noticed when I was at the conservatory, was that there was a restriction for the people playing converter accordions... they had to take the time to physically change registers to change "systems" where all I did was raise my hand higher and I was in "free bass land" (lol). Not often, but in some songs, this was a big advantage. Where they had to change registers, causing delays in the music, I did not have to and the music flowed without interruption.
Also, it may be sincerely silly to say, but I love the look of all those buttons. I cannot tell you how many times I had people stop and stare at the left hand side of my accordion when I was playing, because of "wow, look at all those bass buttons!!!". That coolness factor cannot be beat... lol
It wasn't until I got the Roland that I had my first real taste of a converter system, and the convenience of having the buttons lower was felt, but even at that, I absolutely hated it because the buttons were so far apart (nothing to do with converter vs MIII), it caused my fingers to cramp and hurt, but the counter advantage to this system was the ability of having 4 rows of free bass became evident. Now, did I ever use that 4th row... no, not ever in a playing situation, but then again, I never was taught or practiced with it, so there was no reason to use it. Yes, it is an advantage, however, again, only in very rare circumstances.
In the end, the advantages of either system outweigh the other side and I think that if one is keeping careful score, the outcome is an exact tie.
For me, if someone said "here is a million dollars, order the accordion of your dreams", it would *not* be a converter system, but it would be the traditional MIII system with the deepest notes at the top, matching the format of any right-side keyboard. Obviously, there is no logical reason for this, it is all entirely personal preference.
More importantly, if this is your desire, get a free bass... ANY free bass (MIII, converter, C-system, B-system, Quint, Russian system, Moschino, etc... it doesn't matter!), and enjoy all that it offers, that is the more important step to take. The choices you make after that are going to be personal preferences, and I strongly believe, no matter what you do, that you cannot make a wrong decision after that, no matter what those choices are.