We can visualize some geometry here using some highly appropriate ASCII art (-, /, and \ are screens, * is the user):
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*
vs.
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/ \
*
The first one requires you to move your neck much more than the last option; it also provides a better peripheral view. I have a corner desk, so my setup looks like:
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/
*
More or less; I also have a chair that pivots, so I can change my focus quickly without moving laterally. What is really painful is when, to reduce horizontal span, the monitors are thrown into portrait, then you have up-down neck strain...which is very painful! I actually decided against another 24" as my second monitor for this reason (I have a secondary 21" wide screen in portrait as my secondary, nice for reading PDFs and email).
I would prefer to have one wide monitor, but with a curve.