I don't have a strong opinion either way, but I do have to laugh at "use the old BG system" thing, because all that really changes is where we click and drag.  If you wanted to move characters individually, you still had to issue a group wide hold command to do it, which is essentially unchaining them...
Except the bg1 2 characters stay where we leave them, have controlable portaits, with out clumsily trying to disconnect them from each other every time you need to micro them 
Bg1 party system  is superior  to the DoS one in most ways
6 slots  is better then 4 
Multiple npcs for lots of classes, (and before you say ha early access) - I will be very shocked if we have more then 1 joiniable npc of each class type (if that) 
Bg had lots of the same class with diffrent personalitys so you could actually  role play and shape the personality  of your party, and with it your play style
And finally why 6 is better then 4 imo
In a party you probably  want a tank,  healer, Charisma class, spell chucker, a theif,(locks traps)
So to cover the most roles you will probably  have to play a Paladin to cover tank healer  Charisma
Bard again to charisma healer theif
Or sorc to charisma caster
Druid, tank healer
That leaves you 2 classes to cover if your paladin of bard, or 3 if you are sorc,  or druid
So if you are paladin, you need astarian, in bg2 you would use a good theif like imoen.
You need gale  to cast spells
a bard may support the group better  then  astarian (ill assume we will get an npc)
Bad could be better role play wise  and extra you may need the extra healing  with the paladin
1 class left for whatever is needed (probably  and exta mele like the barbarian, since a paladin and lazel would not get on imo.
Thinking about this makes me feel how restrictive  party classes vs  npc personality is.
I realise that having 6 party members does not effect current  npc personalitys, but it does get rid of the need to have  each class cover 2-3 rolls just to be a good adventuing party, and this will allow for us to pick followers based more on personality then a role we cant fill otherwise
This is why i said larian need to use the old bg games as inspiration instead of DOS, because  bg1 and 2 just do party  systems better