^ Bingo nailed it! B6 would have more shock extension (droop) available.
Less droop with B8s on extension, For all intents and purposes this only places the shock closer to the "middle" of it's available travel at ride height when using the same lowering springs on the B8, versus B6. In general more droop a better thing to have than the opposite of not enough; it allows the tires to stay in contact with the ground for a longer duration until full extension.
The biggest concern with too much droop, is if the spring is not long enough on full extension it wouldn't be seated, that's why helper springs are used in coilovers for instance, to keep the main spring always seated. With aftermarket springs it shouldn't 't be any problem if they were "designed to be used with" the stock struts in the first place; so an aftermarket shock of same OEM length (B6) is fine. No need really go shorter shock (B8) unless you want to be in the "middle of travel" and give up droop.
Saying this as someone who has B8s and super happy with them but would go with B6s if had to do it again.