No publicly available drive/firmware of any brand can burn -R DL at the moment. -R DL media does not publicly exist. And as far as I'm concerned, -R DL is nothing more than a vaporware at the moment.
Use +R DL if you want to burn DL. There is absolutely no advantage whatsoever to using -R DL. It will be less compatible (as compat. is based on time-on-market), less readily-available (and thus probably more expensive when it comes out), and none of the first-generation DL writers will support it. There simply is no reason for anyone to use -R DL except to pay homage to the lame duck -R camp.
Yea, I'm a bit biased against -R, but the way I see it, they've long ago dropped the ball. While +R offered technical superiority when they created the mess of a competing format for single-layer recordables, -R is offering absolutely nothing by offering a competing format for double-layer, so why on earth they are wasting people's times and messing things up with a new format is beyond me, and I see it as a stupid, self-serving, and anti-consumer move on the part of the DVD Forum.