I would never pay more than it books for, KBB's values are for excellent condition, anything less I deduct for. Sounds like the cheaper one with 15K on it has had less than perfect care and maintenance, I huge red flag in my book.
Other bike, garage kept, low miles, great condition. To me that is a no brainer. Spend the extra grand for peace of mind, I bet it ends up being worth every penny.
"Where ever you are, there you go..."