In short, the more upright the bike is the better in general.
The suspension is designed to work vertically, not horizontally.
You are less likely to scrape hard bits.
Tyres generally work better when not leant right over.
You have a greater margin for error or safety, should you need to make a correction or alter course to avoid something or should say the corner tighten on you.

So lean with the bike or kiss the inside mirror.

That said, you have plenty of ground clearance on the XT and certainly leaning the bike under you allows for quicker turning where necessary, so there are times where I use this method too.