Best Language for the job?

The problem is simple.
given a binary string of X(several megabytes)
it needs to be XORed with binary string Y(the same lenght).
The strings are read(converted) from files,and then results written to third file.After which a shell script is
performed on the third file.

The question is:Which language this problem is best coded in? (best=least code size,and the solution must work on Gigabyte+ files)

>>26
ByteString does quite a bit of unrolling and fusion and all that jazz. I'd worry more about the bit where you don't use "B.putStr" instead of the map thing.