Yes, you'll get the feeling of more power down low (with only the primary open) with the 34/34 but at a small mileage penalty. I would imagine the mileage difference is minimal. The power difference is probably minimal, too at WOT between both carbs. I suspect it really all depends on the venturis.
If you where in the states I'd say go with the 32/36 since they are more common here. However, since the 34 came stock on many falcons down there, and the parts are plentiful, you will probably be quite happy with the 34/34. Either way, I think you can easily upgrade to the 38/38 if you decide you need more power since they use the same mounting flange.