In terms of RAM speed, one thing is certain: smaller memories are always faster. However, in your case, since we are talking about 8-bit and 16-bit in relation to the same memory size, 512MB.. the difference in performance is not significant.
The number of chips is more significant to performance. Most motherboard chipsets prefer double-sided modules. So between 8-bit and 16-bit modules, I would choose the double-sided setup.
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