I heard the combined roll in an order66 episode. I liked it, made a lot of sense for anything your players are doing as a group.

Like, is your group aware of the ambush? Can your group outrun the enemy? Etc. However, separate rolls make more sense when your group is doing things as individuals. Can your group track the enemy? Well maybe, but it's probably due to one guy.

In a situation where each player rolls against the same dice pool I would say the pool stays upgraded. You could also do it with every player rolling their positive pool and you roll the negative pool. Get results that way. Takes less time, requires more dice.