7750 has higher Memory Speed, Shader Processing Units, Actual Shader Performance, DirectX, Open GL, Max Resolution (WxH)
550ti has higher Core Speed, Memory Bus, Memory Bandwidth, Texture Rate, Pixel Rate, Render Output Units, Pixel Rate, Max Power
source:
http://www.game-debate.com/gpu/index.php?gid=797&gid2=483&compare=radeon-hd-7750-1gb-gddr5-vs-geforce-gtx-550-ti
in other words, 550ti is better in gpu core, memory and display but 7750 is better in supports DirectX 11.1 (vs 11), Open GL 4.2 (vs 4.1), Max Resolution 4096x2160 (vs 2560x1600). hope that helps.