Chromatic button accordions can have the notes arranged in various different layouts. The two most likely layouts are the imaginatively titled B system and C system. Each system is respectively named after one of the notes found in the first row of the keyboard (the row nearest the outside of the keyboard, furthest away from the bellows). Certain systems tend to be favoured in one or another country.
The Russian Bayan uses the B system layout for the right hand keyboard as do so called Norwegian system accordions and they are more likely to be played in eastern Europe (for example Ukraine and Poland). C system accordions tend to be played more in western Europe (for example France, Italy and Sweden). Here in the UK there doesn’t seem to be a clear favourite yet with players or teachers usually championing the system that was recommended by their own accordion teacher.
For accordions with free bass keyboards, the left hand system usually, but not always, goes hand in hand with the right hand system. The keyboard on a C system free bass accordion is usually a mirror image of the right hand keyboard which means the low notes are at the top near the player’s chin, with the low notes near the floor. On a Russian B system, the left hand keyboard is arranged as if you are playing the right hand keyboard but upside down which means the low notes are near the floor and the high ones near the chin. The Norwegian B system also uses a mirror image of its right hand keyboard for the left hand keyboard. For converter or standard bass accordions, the left hand keyboard is usually the same irrespective of the right hand layout.
Many opinions are offered on the benefits of one system over another. In my personal opinion (I play a Russian B system accordion by the way) there isn’t much to choose between them. If good fingerings are chosen, scale type passages usually put the hand and wrist in the same position regardless of whether you are playing a C system or B system accordion and chord and arpeggio type fingerings are often identical for each system. For me, the main concern should be how easy you will find it to learn and that is more likely to be governed by questions that don’t involve debates about which system puts your hand at a 1 degree more “natural” angle than the other. If you are learning on your own (or with a teacher who hasn’t much experience of chromatic button accordions) can you obtain suitable tutor books, or other material, in your language? If you need the help of a teacher, does that teacher have experience teaching it?
If however you are looking to play a chromatic button accordion with free bass, whether that is now or in the future you should consider one technical aspect. With Russian B system accordions, the fingers go in different directions to the notes, i.e. when the hand is held against the keyboard, the thumb plays lower notes than the fifth finger on the right but higher notes than the thumb on the left. The other systems (C and Norwegian) both have notes going in the same direction in both hands. The Russian system can seem more logical to anyone familiar with a Piano or Organ keyboard for this reason but having said that many ex-pianists have no doubt gone on to be great accordion players whichever system they chose to play.