Which are better and why ?
There’re no ultra metallic (at least i don’t know them).
TDK Reflex Metallic 16x are made by Taiyo Yuden, are very high quality, but can’t be burnt faster than 16x.
16x Reflex Ultra are Ritek, but they seem to be good, too. Can be burnt faster than 16x (I made 24x with a Plextor writer)
24x Reflex Ultra are Taiyo Yuden and very high quality.
32x Reflex Ultra are TDK, seem to be lower quality than Taiyo Yuden (note that most other discs are of lower quality than Taiyo Yuden), but OK.