I would personally go with the two 512 sticks, for dual channel configuration.
Google found this
Quote:
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It's preferable to have paired RAM in the Mac Mini and MacBook, because having a pair of identical modules allows the motherboard chipset to run in dual-channel mode. This is particularly useful for these particular models because the video circuitry steals a substantial hunk of memory bandwidth. There's a measurable performance loss when running with different-sized modules.
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