xbit is close to XBT because it would be the same unit, both would mean 100 satoshi or 1e-6 Bitcoin.
xbit would be for everyday use, XBT for ISO.
I know, the XBT was used by some sites to be a synonym for BTC that is however in my opinion not yet graved in stone until it is used by e.g. Bloomberg.
"xbit" is only a typo or spelling error away from "XBT", and some folks may assume they refer to the same unit of measure, not knowing the new currency system as developers here do.