I think all your problems with UM will be solved at Tufts like the campus' size, the class' size, the school spirit on something not sports and Medford is a small town right next to Boston. If you decided to apply to Tufts as a transfer student you must have known about all these facts, Tufts is perfect for your need. But to choose between Tufts and Northwestern it also depends on what you want to be major in.
Also the big difference between Tufts and Northwestern is NU has the quarter system and all the undergraduates would need to complete 45 credits to graduate. I believe the requirement is much lesser at Tufts, depend on the school you get accepted to: Engineer or Art&Science.
If you need one more random reason, I would say I love Tufts and I think you would too.