Unless you are trying to shoot close-ups from long distances, the 55-250 will give you more of the FoV you would typically use. The 55-250 lens on your T3i has the FoV of an 88-400mm lens mounted on a FF body. If you need wider FoV, this works against you. If you need the extra reach this works to your advantage. A 70-300mm lens on an APS-C body like your T3i will give the equivalent FoV of a 112-480mm lens mounted on FF body. Although the focal length of a lens is the same regardless of the sensor size, the Field of View (FoV) obtained will be different. The difference between "full frame" and APS-C sensors. The 55-250, while not outstanding, performs comparably to the more expensive EF 70-300mm f/4-5.6 IS USM in terms of optics. In the case of these two lenses, the 55-250 can use smaller amounts of higher quality materials and still be manufactured for a very reasonable cost. Another benefit is that some of the lens elements themselves can be smaller, and thus use smaller amounts of the expensive materials used in modern lens design. Since the APS-C image circle is only about 40% the area of an EF image circle, this allows the lens design to be smaller and lighter. An EF-S lens must only project an image circle large enough to cover the Canon APS-C sized sensors that are approximately 22.5X15mm.
AN EF lens must project an image circle large enough to cover a "full frame" sensor or film negative (roughly 36X24MM). The difference between EF and EF-S lenses. There are several reasons why this is so: In my opinion, at half the price, it would be better value for you.
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