North tower is further away from the main casino and I believe the rooms are also a little older. That's where you'll find the cheapest rooms ($25-ish a night during the off-season).
In reality, you can book a room wherever you want to suit your needs/taste/budget, but the group rate will very likely be for the north tower only (and again, it's probably only going to save you a couple of bucks off of the price, because that's basically their minimum price).