Set up camp on the good site with the tent easily visible from the lake, maybe hang some laundry up to dry. Then quickly paddle over to the lesser site and sit on the rocks near shore. When the big group comes by, they will see both sites as occupied and move on to the next lake, leaving you the good site AND peace and quiet.

;D ;D ;D

Just kidding (maybe I'll get flamed again).

I take the good site (on the first come first served basis), and come out ahead if the other group is quiet (there are some like that) or if they weren't planning to stay at that lake anyway, and they move on. If the other group is noisy but move on the next day, then I still have a chance at one quiet night at the lake I planned for.

When I planned my trip, I could see on the map that the first lake had two sites close together, and that I would probably have neighbours for either one or two nights. Knowing there is a big group behind me has changed the odds a bit, but probably not much, and I must have had good reasons for planning to stay two nights on this particular lake in the first place. If solitude was that critical, I wouldn't have planned to stay here at all. So why change the original plans?