I prefer Bar cos it's basically the metric kPa (certainly the difference in conversion is much much smaller than the error range of any of my pressure gauges) but within an easier numeric range (typically 0.8 - 2 bar vs. 80 - 200 kPa). Mentally converting back from PSI when required is dead easy anyway (again within the error range of my gauges) - just divide by 1.5 and shift the decimal place (i.e. 15 PSI / 1.5 = 10, shift the decimal place = 1.0 bar; 12 PSI / 1.5 = 8, 8 / 10 = 0.8 bar; 45 PSI / 1.5 = 30, 30 / 10 = 3 bar, and so on...)