Believe neither. Any hi-freq. measurement should be suspect. Every probe
will affect the signal and its display in some way.
Are you probing the same exact two points, and are both probe leads VERY
short? Probing a signal and ground pin that are only an inch apart from
each other, can introduce sizable errors.
The differential measurement *ought* to be better, *if* the diff. probe is
sufficiently accurate and wide bandwidth, as it avoids the ground loop.
Try doing a differential measurement with two FET probes, and see how it
differs from the other two. Try various combinations to get a feel for how
these changes affect what you measure.
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