Here's one, but it's not quick and I can't think of a way to make it quick, but maybe someone else can.

CREATE FUNCTION FindDifferenceCount(@string1 AS VARCHAR(50), @string2 AS VARCHAR(50))

RETURNS TABLE

WITH SCHEMABINDING

AS

RETURN(

WITH

N(N) AS (

SELECT 1 FROM (VALUES (1),(1),(1),(1),(1),(1),(1),(1)) N (N)

) -- 8 rows

, TALLY (N) AS (

SELECT ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM N N1, N N2

) -- 64 rows (input strings are 50, so 64 is more than enough)

, LETTERS1(L,P) AS (

SELECT SUBSTRING(@string1, T.N, 1), ROW_NUMBER() OVER(PARTITION BY SUBSTRING(@string1, T.N, 1) ORDER BY (SELECT NULL))

FROM TALLY T

WHERE SUBSTRING(@string1, T.N, 1) LIKE '[a-zA-Z]' COLLATE Latin1_General_BIN

) -- Select only the letters and an ordinal from @string1

, LETTERS2(L,P) AS (

SELECT SUBSTRING(@string2, T.N, 1), ROW_NUMBER() OVER(PARTITION BY SUBSTRING(@string2, T.N, 1) ORDER BY (SELECT NULL))

FROM TALLY T

WHERE SUBSTRING(@string2, T.N, 1) LIKE '[a-zA-Z]' COLLATE Latin1_General_BIN

)

SELECT DISTINCT

CASE -- Find the largest number of differences

WHEN COUNT(R1.L) OVER() > COUNT(R2.L) OVER() THEN COUNT(R1.L) OVER()

ELSE COUNT(R2.L) OVER()

END AS DIFFERENCES

FROM LETTERS1 R1

FULL OUTER JOIN LETTERS2 R2

ON R2.L = R1.L

AND R2.P = R1.P

WHERE R1.L IS NULL OR R2.L IS NULL -- We only care if there is no match

);