[sdiy] 2 multimeters showing different voltages

[Scott]

I've got two multimeters, and each is showing a different voltage.  One
shows my synth to be at a solid 15 volts, and the other shows it at
17.5.  

I tested my computer PSU to try to figure out which one is correct, and
I still don't know!  One shows the PSU 12v output at 11.05, and the
other shows it at 12.62.

Both meters are less than 3 years old, and have little use.  One is a
radio shack, the other is a Velleman.  
I'm starting to think that the one that shows 17.5v (and 12.6 on the
computer PSU) is correct.  However, that would mean my synth PSU came
calibrated incorrectly.  (which is possible.. its just a used power-one
supply)
Does anyone know how I can figure out which to trust?  

Also, how much of a difference would those 2 volts make?  If my synth
PSU is putting out +/- 17.5v, would that explain some tuning issues I've
been having?

thanks!