I also use SELECT NULL.
Personally if something is faster and there's virtually no complexity difference, I use the faster version. Why waste where there is no need is my motto.
The point is, it's not significantly faster, it's a couple of nanoseconds at best. It's micro-optimisation, it's a waste of time for no value.
Use EXISTS (Select *, EXISTS (Select 1, EXISTS (Select NULL, or any other form as they are not going to make anything resembling a noticeable difference.
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: Discussions on DB performance with occasional diversions into recoverability
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