If you dont know what mt rainer is you dont really need it. Its just a way of using a CDRW like a floppy disk.
Bitsetting is extremelly useful if you are backing up DVD films only. If your using it for data backup its really not usefull to you. All Bitsetting does is changes the booktype of DVD+R DVDs to be read as DVD-Rom Disks, this means if you have a standalone DVD player that wont play DVD+R or DVD-R disks, it will now by this. You can make the NEC 3540 do bitsetting by upgrading the firmware (its currently not an official firmware though). It works great as i have it on mine.
What i would honestly suggest is you buy both a DVD burner and a DVD-rom drive, you machine should have 2 drive bays. You can have 2 drives on you machine. The majority of people have one burner and one DVD-Rom. You can pick up a DVD-Rom for next to nothing Â£7-13 here in the UK. Try Ebay people sell them for pennies on there. I saw a used liteon go for Â£1.50 a couple of weeks ago.
I would recommend personally through experience either the liteon or Aopen DVD Rom and The NEC 3540 burner. You are fully covered then for all aspects of your needs.
If though you are concerned about reading ability, and bitsetting out the box. And you dont want to buy a DVD-Rom. And also want extrmelly good quality burning then your absolute best option is the Benq. (in the UK its also one of the cheapest DVD drives you can get). The 1625 is good as it does litescribing too. (However the Benq has slightly less media compatability for unkown brands of recordable disks than the NEC has.
Really its up to you what you want to do more with your drive which one to get. As they each have specific strengths
This should hopefully help