The tilde seems like a vague symbol for me. What's wrong with a symbol that includes the letter B?
Nothing, except that these symbols aren't readily available on a keyboard. The tilde is accessible on any keyboard, even phone right now.
At certain point none of these symbols were available on a keyboard. I still don't have an Euro symbol on my keybard, and as you can see I can use it quite well by typing 0128 while pressing ALT. If Bitcoin will be known enough symbol for it will be there when you will need it, for now it is too early. As even wider known currency symbols are not incorporated...