I am currently scouring this site with almost this same question. I am actually keen to know the practical difference between a 2-row (with two accidental buttons) vs a 2.5 row. I am hoping to make a GC purchase that will last for years and I don’t mind spending a bit extra if I am getting value for money...
As Jeff/Melodeon has said, it really comes down to the sort of music you want to play.
The basic practical difference between a two-row with two accidentals and a 2.5 row is that the latter gives you the accidentals in two useful octaves (as opposed to only one octave in the former) and usually a couple of useful reversals as well.
The jump-up in weight between a 2-row 3-voice box such as a Hascy (3.9 kg) and 2.5 row 3-voice box such as the Mory (5.4 kg) is significant and needs to be taken into account. There is a smaller increase when you go to a full 3-row 3-voice box such as the Handry 12 (5.6 kg), but you need to decide for yourself whether the extra weight and cost justifies buying one. Personally I find the 2.5 row Mory has exactly what I want and no need for any more extra buttons. But your own needs and aspirations may well be different.
One other point - comparing like with like in terms of layouts and playability, although they are both very fine instruments, the Beltunas are noticeably heavier to play than the Castagnari equivalent.
Ultimately it comes down to personal choice and 'try before you buy' is perhaps the best advice. Also, you are unlikely to immediately find the perfect box that will 'last for years'. Perhaps start somewhat modestly with a 2.5 row and play it for a a year or so and then, if you find you
really need all the extra buttons on the third row, trade it in for a full 3-row box.