pgpool2 setup on master-slave replication using postgreql 9.1
Of PgBouncer and PgPool-II, only PgPool-II supports load balancing.

PgPool-II's load balancing supports only read-only queries that meet certain requirements; see the pgpool docs. Additionally, if you start load-balancing queries you are no longer guaranteed to get a consistent view of the database because the replica might be lagging behind the master.

Personally, I'd look into adding a caching layer (memcached / Redis) before doing fiddly things like read-query load balancing.