GTX 550 Ti has Fermi 2nd gen while GTX 650 has Kepler 1st gen.
Kepler is more efficient but the 2nd gen (which is used in GTX 780) is better (because of it's performance).
Compared to 2nd gen Fermi, 1st gen Kepler is almost the same.
Except the texel rate, the 550 Ti wins hands down.