XBT makes the most sense and should be used all around.
BTC is Bhutanese coin because BT stands for Bhutan.
X is allocated for all types of currency (and some commodities) that do not belong to one specific country. XBC is already taken, so XBT would make the most sense.
So, everyone please stop using BTC and start using XBT.
For clarification, the first two letters are the country code: US(D), EU(R), GB(P), BR(L), except for currencies/commodities that are not specific to one country, they use only one letter, specifically the X, and no country can ever start with X.
X represents the Bitcoin cause very well, because it is international. Similar global currencies include XAU (gold), XAG (silver), XDR (international special drawing rights), etc. The letter is familiar to many in the financial community as an international token of value.