Sprint goes up to a higher tension so it's harder to clip out. In my opinion, you don't need that unless you're riding the velodrome or are a sloppy but very strong road/crit sprinter.
Sidi and Shimano are decent shoes. The lower end models have fewer straps and softer soles, so they're a little harder to fit comfortably and they lose a tiny bit of energy on the pedal stroke. More importantly, Sidi and Shimano fit differently from each other. I fit Shimano shoes but not Sidi... Sidis are too narrow for me, and their Mega line is too wide for me. Shimano fits just right.
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