買 D9 邊隻牌子好?

You have following way to choice.
1) Compressing your D9 to D5, then, you could burn it on D5 disc but with quality lost. There are many software could do it. The following is one of it.
http://www.elby.ch
2) Divide the D9 disc into two D5 discs. The quality would be remained. However, you need two D5 discs. The above software could do it too.
3) You could use D9 disc actually. Mitsubishi is good. It is around $18 per disc for a pack with 10 discs. However, Verbatim quality is closed to Mitsubishi but a little bit cheaper. It is around $ 16.5 on 20 discs. If you don't use so many, Mitsubishi is recommended. You could find them in WanChai 298.

D5 碟燒不到 D9.

DVD-5
DVD-5是DVD論壇定義的規格，為容量4.7GB的單面單層DVD片（1GB等於1,000,000,000 bytes，而不是慣用的 1,073,741,824 bytes，下同）。


DVD-9
DVD-9的規格為容量8.5GB的單面雙層DVD片。

reference : http://zh.wikipedia.org/wiki/DVD