USM is Ultrasonic Motor, ie the motor in the lens which controls autofocus. It probably won’t have much effect on your sharpness, other than that autofocus accuracy might be a bit more accurate. What it will give you is fast, silent AF. Also, USM lenses (though I think not all of them) can pass the focusing distance to the flash to help flash exposure, and will also allow manual override when AF is engaged (not a good idea to try this on a normal lens). I think the latter two may only apply to the ring type USM