Mostly has to do with carcass and compound: the Bridgstones are an OK street/track tire for the casual trackday person; they heat up quick and give decent grip (good for the street); but if you are into serious track times then you are going to be sliding around a lot.
The Michelin Pures are a softer compounded tire, but slower to heat up (they need more flexing to get hot), but they stay stickier, longer. If you get to a serious pace, you will see some tearing, but that is normal. Tires are good for a sprint race or two, depending on abrasiveness of the track and pace of the rider, but I've seen the right side of Pures destroyed in one heat at Thunderbolt by aggressive riders.
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