Being that Bitcoin is decentralized, there is no standard, nor governing body, to dictate what notation should be used for it. Nonetheless, "BTC" has been the generally accepted abbreviation for Bitcoin stemming from the early days of Bitcoin.
"XBT" is a new abbreviation for Bitcoin that is starting to come into use, and reflects its growing legitimacy as an international currency.
The abbreviation "XBT" comes from the International Standards Organization (ISO) that maintains a list of internationally recognized currencies. The standard dictates that if a currency is not associated with a particular country, it should begin with an “X”, hence "XBT". Another example of this is the abbreviation for gold, "XAU".
In other words, both are valid, but XBT will most likely become the norm eventually.