12x DVD-R DL
12x DVD+R DL
It can write at speeds lower than those above, down to 2.4x for DVD+R/ DLDVD+R and 2x for DVD-R versions.
Do you need an External USB drive though?
Most computers in the last few years have an internal (IDE/SATA) DVD writer, which should be about 1/2 to 1/3 the price of an external.
Reliability wise, USB writers may have trouble reaching full speed.
If you don’t need lightscribe (burning the label on special [aka expensive] lightscribe CD/DVD’s) you might save a few $ by getting a non-lightscribe version.
The only reliable Dual Layer DVDR’s are verbatim.
The single layer DVDR’s that come recommended, go with Taiyo Yuden (f you can get it) or Verbatim if you can’t.
If it’s a bargain, it’s probably too good to be true