I agree with other posters here that say you will not get -R DL on the GSA4163B, although LG is rooted in the -R camp. They never did enhance their drives with a firmware upgrade, only update media compatability. It will be a big surprise for everyone, if that changes.
This is not to say the burners are no good. I own a 4163B and am very happy with it. People owning the NEC 3500 and other of the past NEC burners are usually also very happy with it. You get tons of modified firmwares for the NEC, very little of this stuff for the LG. The 3520 differs from the 3500 though, but some guys have already come up with firmware hacks. I haven't read anything about general satisfaction with the 3520.
The NEC does not support DVD-RAM. If you need it, you don't want to buy NEC. Accordingly, if you really need -R DL and can't survive with +R DL, you don't want LG.