I'm probably the wrong person to ask.
5 rows (6 would be interesting too I guess) for the "modern" fingerings. The traditional 3-row mentality makes it very hard to learn & play. I'd say that people who have achieved greatness on 3-row fingerings were able to do it not due to any benefits offered by the 3-row system, but due to their great musical talent & hard work, despite the limitations of the 3 rows.
That's where my expertise as a player ends - I'm just a beginner, really. But, after a decade as a guitar luthier, I like to think about the instrument's (and not the player's or composer's) capabilities even if my playing skills will never reach them.
With almost every instrument out there, there's a "natural" range that sounds best, and there's the peripheral range that still plays a note, but most of the instrument's "magic" is lost. With guitars, it's trying to add extended bass strings all the way to low B; with the mandolin, it's playing up the neck; with the clarinet & flute, the low & the lower middle registers really sing, but the "mastery" of the instruments implies that you're pushing the high range to the limits with those horrible screeching high pitch sounds... With the accordion it seems to be the desire to cram as many notes as humanely possible into the box at the expense of everything else - tone, response, tuning, dynamics. I'd take 3 very sweet sounding, well set-up octaves over 8 any day.
We seem to live in the world where music is often treated as a sport - you've got to play faster, with an increased range, push the instruments beyond the range they were designed for; force your instrument of choice to perform music written for something completely different instead of opening up its true nature. The professional performers' ability to slow down, play what their heart desires and still be respected seems to have to be earned by years of making circular-saw-like noises.
Of course, there's a completely different point of view if you ask a composer - the tendency seems to be to value range over tone ("Give me enough notes to express myself!").
I've been on an accordion discovery journey, and I'm figuring out a lot of things for myself. I hope I'll be forgiven for saying this, but the accordion reed, despite its ability to cover a huge range in a relatively small box, is not the most pleasant sound source. I find that anything higher than C6 in the M voice is too sharp. On non-cassotto and some cassotto instruments, anything below C3 starts sounding suspicious. A very well-implemented cassotto or larger "Bayan-style" LHS reeds easily expand the "sweet spot" to C2. But who really needs that E1 or F1 that sounds like a broken old Landrover engine? So the sweet spot, to my ears, is C2 -C6. Having free bass an octave lower than RHS gives you 3 octaves in each hand, all of them working very well. That's enough notes for me to play until the day I die, but I appreciate others will want more. Another thing to consider is the size & weight of the box - the concert converter instruments seem to be very heavy & bulky, poorly balanced, unpleasant to handle even for a big guy like me.
Definitely freebass in the LHS for me. Not just due to the music choice, but also because I feel that chord bass really robs the dynamics from the right hand. The only times I like to listen to others playing Stradella is when they use it as little as possible. This is about music for listening - if you're talking dance accompaniment, the story is completely different: you want the loud punchy Stradella LHS to keep the beat for the dancers, while the intricacies of the melody get completely lost in the heat of the dancefloor.
Being a dummy, I want the most efficient/easy to learn free bass system, hence my search for a good fit before I commit and start learning one.
My piano accordion with 41 key gives a natural range of 3 octaves plus a major third (F3 to A6) with a maximum extension of F2 to A7 (with switches). On my bass I have 3 octaves starting on C2 to B4 (36 notes). It can be 'moved up' one octave to B5 with a register change. Many people would find this very limiting but I am happy with this range of notes on my PA, because I play folk/trad. type melodies and F3 to A6 (41) pretty much coincides with the range of the fiddle and the LH (36) is similar to the cello range.
