Opticallimits

Still confused about USM vs STM, do both give decent quality for videos ?

I noticed my 18-55 STM outpeforms my 17-55f2,8 for video AF but it's not fair since 18-55 has much greater DOF 

anyone can help ?

If your camera supports full use of STM in video-mode, the STM-drive

will produce smoother AF-operation in video-mode as compared to USM.

 

So, with the right camrera and with the emphasis on video, STM is

likely the way to go.

Exactly, USM is often too nervous often goes from front to back focus before locking on 17-55 while transition is smooth on 18-55

Actually Nano USM is the way to go now ;-)