9
The expressions are the same as long as the operands are strings; if not, + might add them instead depending on type conversions. & guarantees you won't get anything except a string concatenation, and will convert operands to strings if possible to do so.
There's an MSDN entry about Concatenation operations in Visual Basic that explains it:
The & Operator (Visual Basic) is defined only for String operands, and it always widens its operands to String, regardless of the setting of Option Strict. The & operator is recommended for string concatenation because it is defined exclusively for strings and reduces your chances of generating an unintended conversion.