There's no difference if both operands are strings. However, if one operand is a string, and one is a number, then you run into problems, see the code below.
"abc" + "def" = "abcdef"
"abc" & "def" = "abcdef"
"111" + "222" = "111222"
"111" & "222" = "111222"
"111" & 222 = "111222"
"111" + 222 = 333
"abc" + 222 = conversion error
Therefore I recommend to always use & when you mean to concatenate, because you might be trying to concatenate an integer, float, decimal to a string, which will cause an exception, or at best, not do what you probably want it to do.
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