Focusing

• The 75-300mm lens is available in two different models: one has an ultrasonic focusing motor, one does not. The one with the focusing motor, labeled a USM lens, will focus on objects that are far away much quicker than one without. The 55-250mm lens does not have an ultrasonic motor.
  
  

Image Stabilization

• The 55-250mm lens is an optically-image-stabilized lens. As such, it is designated an IS lens. The image stabilizer enables handheld shooting at four stops more than a lens that does not have an image stabilizer. These extra stops make shooting in low light without a tripod or monopod much better. Neither version of the 75-300mm lens has an image stabilizer, so you would need to carry a tripod for low light shooting with the 75-300mm.
  
  

Focusing distance

• Aside from the focal length difference between a 55-250mm lens and a 75-300mm lens (the former has better wide angle, while the latter has more telephoto), the closeup focusing distance is different between the two lenses. The 55-250mm can focus on an object as close as 3.6 feet/1.1 meter away, while the 75-300mm lens has a close focusing distance of 4.9 feet/1.5 meters.
  
  

Glass

• The 75-300mm lens has 13 lens elements in nine groups. The 55-250mm lens has 12 lens elements in 10 groups. One of the lens elements in the 55-250mm lens is a UD-lens element, a higher-quality glass that reduces chromatic aberrations throughout the lens range.
  
  

Lens type

• The 55-250mm lens is an EF-S lens mount, meaning it will not work on Canon's full frame D-SLRs like the 5D. Check for the compatibility with your camera model before purchasing a 55-250mm. The 75-300mm lens is an EF lens mount, so it will fit on all Canon SLRs.