Hardtail: 650mm
Fullsuss: 700mm
I have a shorter stem on the Fullsuss, which compensates slightly for the wider reach, the wider bars are greater for techincal stuff, especially downhill techincal, and the wider bars also allow you to get the brakes levers further in to the centre which makes one finger braking easier. The wider bars sometimes present a few problems with narrow gaps, some dodgy moments when I have clipped a few trees. If the bike was staying mainly on the road, I'd use about a 600mm wide bar, and a slightly longer stem, but if it's going anywhere off road and techincal, short stem and 650mm plus bars.