SET is the ANSI standard for variable assignment, SELECT is
not.
SET can only assign one variable at a time, SELECT can make
multiple assignments at once.
If assigning from a query, SET can only assign a scalar
value. If the query returns multiple values/rows then SET
will raise an error. SELECT will assign one of the values
to the variable and hide the fact that multiple values were
returned (so you'd likely never know why something was
going wrong elsewhere - have fun troubleshooting that one)
When assigning from a query if there is no value returned
then SET will assign NULL, where SELECT will not make the
assignment at all (so the variable will not be changed from
it's previous value)
As far as speed differences - there are no direct
differences between SET and SELECT. However SELECT's
ability to make multiple assignments in one shot does give
it a slight speed advantage over SET.