Either one is fine, I've been burning since the early 90's, back when 2X is considered fast. I still have an external Yamaha 400TX SCSI burner (4 times CD-R, no RW) and a defunct Yamaha 2X burner. I also owned Plextor 8/4/32A as well as my current 12X SCSI plextor burner (in my signature).
If both units cost the same, have similar burning rating, and similar software software (extremenly important in my opinion, when I purchased my plextor 12X burner, I had to go back to EZCD 3.5D just so I can use my burner), I will go with plextor. Other people might think differently.
Both brands make excellent burners, plextor also makes excellent CD-ROM drives. I have the old plextor 6X CD-ROM, 8/20, as well as 40X UW SCSI CD-ROM. All of them are still working, but who wants the 6X cd-rom now?
Back when my 8/20 CD-ROM drive won't accept CD-RW media (because it's too old), I called plextor tech and they even shipped a new firmware chip to me (since I can't flash 8/20 firmware). Yamaha also has good tech support.
I don't do too much "backups" since I've been working for a while. I am a programmer so...I try to support fellow programmers I think my stance might change once I start to have children, I've heard some horror stories from co-workers and I think backups are definitely needed, especially with children around. I love the plextor because I used to spend a lot of my HDD space as well as my time in compiling my favorite CD collection. I hate all those audio CD I bought that has only a few tracks I like. I used my plextor drives to make quite a few favorites collection. If you are not into that (audio editing), go with Yamaha if it's cheaper.
Good luck, if it's not for my wife, I would've upgraded more parts of my system