I'm not sure which one you will get either.
As for the microfin one... I think it's slightly better as I have tried my 2400+ with the thicker fin heatsink and get slightly higher temps (not sure if it's enough diff to worry about).
I think it has to do with the total surface area for heat dissipation. The microfin coolers have many more 'fins' so that makes for more surface area.
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