Maybe try swapping the seat from your Mongoose to the Trek? Just the saddle, not the post. It could be that you are so used to the saddle, and of course it's well broken in to fit you, that it will make a huge difference. If so, and the old saddle really could use replacing, ask the shop for something similar and if they will give you any trade in for the original from the Trek, since it should have virtually no wear.