+ operator

String s = s1 + s2

Behind the scenes this is translated to:

String s = new StringBuilder(s1).append(s2).toString();

Imagine how much extra work it adds if you have s1 + s2 here:

stringBuilder.append(s1 + s2)

instead of:

stringBuilder.append(s1).append(s2)

Multiple strings with +

Worth to note that:

String s = s1 + s2 + s3 + ... +sN

is translated to:

String s = new StringBuilder(s1).append(s2).append(s3)...apend(sN).toString();

concat()

String s = s1.concat(s2);

String creates char[] array that can fit both s1 and s2. Copies s1 and s2 contents to this new array. Actually requires less work then + operator.

StringBuilder.append()

Maintains an internal char[] array that grows when needed. No extra char[] is created if the internal one is sufficiently big.

stringBuilder.append(s1.concat(s2))

is also performing poorly because s1.concat(s2) creates an extra char[] array and copies s1 and s2 to it just to copy that new array contents to internal StringBuilder char[].

That being said you should use append() all the time and append raw strings (your first code snippet is correct).