I wonder if we can answer this without starting a wide bars argument.
Basically, wider will give you a bit more control, possibly at the expense of climbing ability, and it’ll stop you fitting through gaps in trees easily. Most people find more than about 780mm feels stupid, but there are people who feel anything wider than 720 is unpleasant. Best option is to buy some wide bars (780mm ish) and some lock-on grips that you can slide up and down the bar. If you’re average sort of height, start with the grips at full width and ride for an hour or so at least, see how it feels, then move them inboard 5mm at a time until you’re comfortable. Once you’re certain, lop the ends of the bar off.
My opinion is that it doesn’t really matter what type of bike the bars are for (it’s more about your physique), although for an XC race bike I’d probably use slightly narrower bars than a trail/DH bike.