XT series is enumerated M7xx, the higher number means latest release/series (like M77x is latest generation of XT, M76x the previous, M75x like you're asking about before that; the last digit differentiates features within the series (might be relatively minor, could be difference between disc and non-disc). There wasn't a 757 hub to the best of my recollection. Check out the various technical documents at techdocs.shimano.com to look at specific differences. I looked a bit at the rear 755 vs 756 and only see a bit of difference in the freehub part number and for an additional washer for the freehub.
755 rear http://techdocs.shimano.com/media/te...9830611910.pdf
756 rear http://techdocs.shimano.com/media/te...9830608935.pdf
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