Mach 1 and I used to get into heated debates about this question. Bottom line is that installed properly, they are both accurate. I prefer mechanical. keep in mind that any resistance at all, either in the wires, sending unit or at the block (too much teflon tape on one of the adapter threads) will effect an electrical gauges accuracy. Also remember that an electrical gauge cannot leak oil inside your car. There are pros and cons to both.
Take care,
-Chris
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