My short answered advice.
If you are going to be using the drive as the reader and burner go with the 811S.
If you are going to be using a DVD-ROM drive as the reader then you might want to go with the NEC 2500A. Although I still would stick with my 811S over the 2500A
Check the great reviews by OC-Freak here.
You also might want to read the last few pages of this sticky at the top of this forum