Performance difference for COALESCE versus ISNULL?
ISNULL is Sybase/SQL Server specific
COALESCE is portable


Then


ISNULL take 2 arguments
COALESCE takes 1-n arguments


Finally, and the fun bit. The result datatype and length/precision/scale


ISNULL is the same as the first argument
COALESCE is the highest according to datatype precedence


This last bit is why ISNULL is usually used because it's more predictable (?) and COALESCE can add unintended data type conversions: which is where the "it's slower" bit comes from

DECLARE @len10 varchar(10); --leave it NULL
SELECT
    ISNULL(@len10, '0123456789ABCDEF'),     -- gives 0123456789
    COALESCE(@len10, '0123456789ABCDEF');   -- gives 0123456789ABCDEF


All datatypes being the same, you won't see any practical difference...