On plextor drives: I loved them, untill they got out of the oem internal dvd market. Now all their DVD drives are just a rebadged drive form another oem supplier. On of the things I liked was their plextor tools for evaluating the quality of recorded DVD's that I backed up. Still have their 716 and their 755 model drives (indaces the high quality as they still work great). For the Blu-ray drives, I'm not sure their "quality" is worth the cost differenial. I have 2 BD rewitable daul layer drives, one from Pioneer and one from LG and both work fine.
Is to the cache size. I think you will find that it only effect burst rate when burning data, so not a real biggy. Only other effect would be if your are doing alot of other task while writting to a disk or watching a dvd while recaculating a large spreadsheet, or coping large amount of data to a HD at the same time. The quality/reliability of the drive would be more of a consideration than the size of the cache. Personnally I have shied away from the Lite-on lineup of blu-ray burners.
Added: Plextor Px-B940 Reviews seem very favorable. But not sure if it is much better than the pioneer version.
Could not find it "made by plextor, or a rebadged pioneer with Plextor firmware.
http://www.ccereviews.com/reviews/plextor-px-b940sa-internal-12x-bd-writer/6/
http://www.cdrlabs.com/Reviews/plextor-px-b940sa-12x-blu-ray-disc-writer/Conclusion.html