A “stability frame” design strengthens the toe and topline while optimizing perimeter weighting for improved performance on mis-hits while a more neutral CG location that’s closer to the center of the face improves performance on center strikes.
There are differences however including most notably more adjustability in the form of Titleist’s SureFit CG technology that allows for more specific spin rate and launch angle tuning. The TS3 also features a more traditional pear shape and tends to produce more of a mid-launch as compared to the TS2’s higher launch.
Optics are critical to putting success — if a putter doesn’t allow you to stand comfortably at address with your eyes in a position to consistently see the target line clearly it’s not for you. Look for a model than sets your eyes either over the ball or slightly inside of it so you can see where you want it to go.