If you want to have full text search capability on Heroku, you should definitely use texticle or acts_as_tsearch. It is possible to use acts_as_farret on Heroku, but since you can only write to /tmp directory and your index file will be deleted sooner or later, you should not use it.
I first used texticle, and later switched over to acts_as_tsearch because I wasn’t happy with performance. Acts_as_tsearch is working really fine for me, and since your database on Heroku is already Postgres, there should be minimal configuration required. I even switched my local database from MySQL to Postgres to make both development and production environments consistent.