SQL: What is better a Bit or a char(1)

a bit and a char(1) will both take a 1 byte to store,assuming you only have 1 bit column in the table, SQL Server will store up tp 8 bit columns in 1 byte. I don't think there is a difference in performance.

One thing to be aware of is that you can't do sum on a bit column

CREATE TABLE #test( a BIT)

INSERT #test VALUES (1)
INSERT #test VALUES (1)

SELECT sum(a) FROM #test

Msg 8117, Level 16, State 1, Line 1
Operand data type bit is invalid for sum operator.

you have to convert it first

SELECT sum(CONVERT(INT,a)) FROM #test