Hello mate, I think that this choice depends by how long you made the vegetation phase. If you like have your harvest more rapidly you could use the 8.5L pot with a faster veg phase ( for example anly 2 or 3 weeks), you'll have plants big enough for your room. So the pot choice depends by differents necessity, i tip one of the master rules for growing every plant: "much is not always better".
Good luck and happy journey!