They all look nice. I have a 66 treadle - & do like it.
I have heard many nice things about the vertical bobbin on a 15. You can get instructions (how to thread the vertical bobbin & care for the machine) on line from Singer - no need to pay for a video copy on ebay. Would love to have your decision to make! Good luck.
BTW, the lever is a reverse for the machine - my Singer 201-2 has same thing.