Whatever make you decide on bear in mind that most airlines (and airports) will be lowering their maximum individual bag weight from 32kgs to 23kgs in early 2008. This lower level will not affect baggage allowances, just the individual bag weights.
So for family of 4 with 20kgs baggage allowance each = 80kgs.
Currently you can go 2 x 30kgs + 1 x 20kgs.
In future you will need 4 x 20kgs.
So my advice is forget the mega large bags and go for no larger than medium sized. Empty weight should be kept as low as possible so that you don't waste expensivve baggage allowance kgs.