Hello Si,
The question of weather is a bit like the roll of a dice. Very hard to predict irrespective of time of the year. The Franklin can rise & fall quite fast (read = overnite), so it is quite possible to have high water trip this week and a low water trip next.
I'd go for the summer half of the year coz of longer day time hours.
Have a talk to Graham Mitchell at Rafting Tasmania. He's a good guy & will give you a totally honest opinion based on having been in the business since 1982.
a.