I own an Uplander and like the interior space a little more than the Caravans, but it's not all that significant. However, having rented both while on vacation, I would say that the Caravan is better for luggage. Since the Caravan has the fold away rear seat, the rear cargo space is huge to allow for the seat storage. Much better than the Uplander's cargo area.
So, I think 6 people will be fine in either, but the Caravan will fit the 8 suitcases the best.