You're in Haskell-Cafe... the bias is there already for Haskell.

Personally, I don't have experience doing network applications in Haskell, but I've got about a year's worth or more using Erlang, so for me it's a pretty easy choice, and I wouldn't pick Haskell today.  

(hearing boos in the background as I type this...)

However, I would invest in learning to use Haskell, because my experience with Erlang has been that it really is pretty great, and has lovely features, like the ability to shell into a running process and poke around if something seems not quite right, or even to ssh into it if you run the right services.  Plus there's tons of stuff pre-written for network applications that just comes with the Erlang distribution, meaning there's nothing else to install.... but even after all of that, I miss the type safety of Haskell, as it's really painful to do this in erlang

function(T1, T2, T3) when is_list(T1) and is_integer(T2) and is_integer(T3) ->

    ....function body

Even if you have a list, you don't know "what of" so you never really get the guarantees that you get when a Haskell program successfully compiles...  That peace of mind is not easily won in Erlang, except by lots of testing.

Dave