I would say sony is the best out of the two becuase it has probably got a larger cache (8mb) but Pioneer 107 is the fastest dvd burner and if Pionner release a firmware update to burn dual layer that will be an advantaged. I would serious wait until a daul layer burner is out.
I'd take the Plextor over the Sony any day. A 2MB cache is in no way a handicap, I went from a Sony DRU-500A (8MB cache) to the Plextor PX-708A (2MB cache) with absolutely no difference in performance. The Pioneer 107 has 2MB cache as well. The Plextor is a higher quality and wonderfully performing drive.