if you really want the dual capability +/- writing then my advice is to go for the Pioneer DVR-A06. This drive is around the £150 mark
You can still get a Pioneer DVR-A05 which only does the -R writing but its a very stable and reliable drive. You can buy it from
here and else where for around the £120 mark.
Both drives write DVDR's at the same speed (x4), but the A06 has slightly faster read times.