I was answering from the point of view of which you should buy, meaning you currently have neither. If you have a 650 Ti then it changes the equation. The expense has to be justified by the jump you get in performance. It won't be.
An RX 460, or 560, or GTX 1050, those are well worth the expense. A new 550 would cost around $100, around here, and you're not going to get $100 worth of extra performance from it over what you're getting now.