Franked, in simple terms is like as if you prepaid tax at 30% of the gross dividend received. That is before the due date of your annual tax lodgment, which you can use as an offset to tax payable. Kind of like PAYG instalments.
Unfranked on the other hand is pretty much like receiving the gross dividend without prepaid tax on it.
For this reason I prefer unfranked dividends as I feel it is like as if I have that money in my hands before having to pay the tax on it and may work better in the compounding long-term if that's your investment strategy. But then again if you receive to much unfranked dividends then you will probably be put on the ATO's PAYGI system anyway.