Hi Paul i read up to 32 values per second. i prefer to put the 32 values in a string and then
write the string to the SD Card.
What you prefer matters very little to the Arduino.
You could look at the source code for the String class to see what the + operator is doing, and what the += operator is doing. There are constructor and destructor calls involved, and the destructor calls free() which, in the version currently used by the Arduino IDE, has a serious bug.
You can continue shooting yourself in the foot if that makes you happy.