My advice would be to pick a language you already know for
this work.
My reason for saying this is that if you don't know the language,
/and/ you don't know web development, then you'l have a hard time
working out what your problems are, never mind what the answers
are. (And your emphasis on performance suggests to me that this
isn't just a learning exercise.)
If you're determined to choose between two languages you don't
know, then pick the one for which you have reason to expect good
help, eg local [1] programmers for/in that language, so that
you can talk [2] problems though and get pointers to good practice.
(But do not take such help for granted and put good effort in to
trying to solve your own problems.)
Chris
[1] Actual local is good, because of refreshment and
whiteboards, but internet-lcoal can be made to work.
[2] "Talk" includes email and IRC and fora and so on.
--
Chris "allusive" Dollin